This study was an in vivo and in vitro virological and immunological study, and was focused on Measles virus vaccinated individuals and in comparison with control group. A total number of 60 apparently healthy individuals were included in this study, 8 were excluded later for various reasons, and a total number of 52 subjects were investigated. Twenty-four of them (46.2%) gave positive history of measles vaccination, the remaining 28 (53.8%) didn't kriow their vaccine status. Nineteen (36.5%) gave history of rash during their childhood, and they didn't know weather related to measles infection or not?. The volunteers were age and sex matched, and were subdivided into two groups. Group 1 included 26 individuals; they were vaccinated with measles vaccine. The mean age group was (30.2) year. 69.2% of them were males and 30.8% were females. Group II (control) include 26 individuals; they were injected with the diluent supplied with measles vaccine (placebo). The mean age of the group was (27.8) year. 69.2% of them were males and 30.8% were females. The subjects of both.groups were followed up for four weeks after administration of vaccine or diluent. Blood samples were taken from all the volunteers prior to and 1, 2, 3, and 4 weeks after vaccination Investigations including delayed hypersensitivity skin test to PPD, ELISA for detection of measles specific antibodies, WBC and differential counts were done for both groups. The potency of live measles vaccine was checked by in-vitro infection and propagation of measles virus in vero cells. Virus isolation technique and immunofluorescence test were done to detect the evidence of viremia during the first week after measles vaccination. Cellular and humoral mediated responses were assessed by enumeration of lymphocytes and functional assessment of lymphocytes, and by study the production of immunoglobuhns, for both groups. A new method has been invented in this study, for cultivation of lymphocytes from whole blood. The established lymphoblastoid cell lines were infected with measles virus vaccine to study the evidence of viral infection. The results were compared and statistically studied for any significant differences. The following results were obtained: 1. Measles specific IgG antibodies were detected in 96.6% of all volunteers' sera as measured by ELISA. 2. There was marked rising in the mean of antibodies after measles vaccination, the OD readings were 1.72 and 1.95 during first and fourth week respectively. 3. Administration of live attenuated measles virus to selected individuals interferes with the capacity of the recipient to express pre-existing cutaneous delayed hypersensitivity to PPD without suppression of pre-existing humoral antibody. 4. Measles virus could not be isolated from lymphocytes of vaccinees during the first week after vaccination; furthermore, measles antigen could not be detected in lymphocytes by indirect immunofluorescence test. While the vaccine used was shown to cause cytopathic effect on vero cells. 5. The administration of live measles vaccine interfered with the capacity of subject's peripheral lymphocytes to respond to different mitogens and antigen in vitro. Whereas, there was little suppression of the in_vitro lymphocyte response to phytohemagglutinin, Concavalin A, and PWM following live measles vaccination, there was a significant reduction in the number of lymphoid cells stimulated in tissue culture by PPD. 6. Total WBC counts of measles vaccinated individuals were similar to that of control group. There was no significant difference between the mean count of lymphocytes, neutrophiles and monocytes of vaccinated individuals and that of the control group. 7. Pure lymphocytes cultures were established, from whole blood and maintained cell division, and lymphoblastoid cell lines were obtained. 8. The present study have* demonstrated that attenuated measles virus can replicate in lymphocytes, as well as in the established lymphoblastoid cell lines. 9. This study confirmed that both stimulated and nonstimulated . lymphocytes became infected with measles virus, but high susceptibility of measles virus infection was observed in the stimulated yniphoblastoid cells. 10.Our findings may explain the role of adequate humoral antibody in control of measles infection.