Two isolates of Lactic acid bacteria (Lactobacillus plantarum and Lactobacillus casei) were obtained from previous study. These isolates were reidentified according to different morphological and biochemical tests. Ability of these two isolates as probiotic microorganisms were examined against enterotoxigenic E. coli (ETEC) in vitro and in vivo. Results showed that L. plantarum propagated on both Man-Rogoza-Sharp (MRS) agar and in MRS broth, for different incubation periods (18, 24, 48 and 72 hr.) under anaerobic conditions had an inhibitory effect against the ETEC grown on nutrient agar plates more than that of L. casei propagated under the same conditions. Results also showed that the optimum period for roduction of probiotic compounds by both isolates was 24hr to inhibit the growth of ETEC in vitro. On the other hand it was found that the ability of these isolates to inhibit the growth of ETEC after the propagation in MRS broth is better than their ability in MRS agar. A different probiotic compounds produced by both L. plantarum and L. casei that may have the inhibitory effect was examined separately. Results showed that the organic acids were the specific compounds responsible for the inhibitory effect against ETEC. In vivo study was carried out on twenty four albino mice Balb/C) randomly divided to 6 groups designated as C, PP, CC, EE, PE, and CE. Each group consists of 4 animals, the obtained results showed the followings: 1. Group C (Control negative): Levels of Glutamic Pyruvic Transaminase (GPT) and Glutamic Oxaloacetic Transaminase (GOT) in the blood serum of untreated animals were 61.25 U/L and 106 U/L for GPT and GOT respectively. 2. Group EE (control positive): Infection of this group with ETEC caused a significant increased (p<0.05) in serum level GPT (99.5 U/L), and a high significant increased (p<0.001) in serum level GOT (270 U/L) in comparison IX with their levels (61.25 U/L and 106 U/L) in group C animals (control negative). 3. Group PP: Dosing animals of this group with L. plantarum causing a significant decreased (p<0.05) in serum level GPT and GOT (36.25 U/L and 90.50 U/L, respectively) in comparison with their levels in serum of control negative animals (group C). 4. Group CC: Dosing animals of this group with L. casei showed no significant differences in the level of serum GPT and GOT (58.75 U/L, 104U/L), respectively, in comparison with their levels in serum of control negative animals (group C). 5. Group PE: Dosing animals of this group with L. plantarum then infection with ETEC showed a significant decreased (p<0.05) in serum GPT level (63 U/L) and high significant decreased (p<0.001) in serum GOT level (114 U/L) in comparison with their levels in control positive animals (group EE), while there is no any significant differences of serum level GPT and GOT of this group in comparison with their levels in control negative animals (group C). 6. Group CE: Dosing animals of this group with L. casei then infection with ETEC showed a significant increase (p<0.05) in serum GPT level (83.75 U/L) and a high significant increase (p<0.001) in serum GOT level (185 U/L) in comparison with their levels in control negative animals (group C), on the other hand, there is no any significant difference in serum GPT level and there is a significant decreased (p<0.05) in GOT level when it was compared with control positive animals (group EE). stopathological study showed that infection of mice animals with ETEC causes a necrosis, nerative changes and inflammatory cells infiltration on some animal's organ tissues (stomach, intestine and liver) as compared with normal sections taken from uninfected mice, while dosing with Lactobacillus plantarum prevents the cytotoxic effect of ETEC on mice X.