Three to five week old male Balb/o mice were experimentally infected with hydalid disease by intraperitoneal injection of 1500 viable E.granulosus protoscolices. The effect of infection on the liver and spleen weights, the cell cycle progression (CCP) and cytogenetics of the bone marrow and spleen cells and adenosine deaminase (ADA) activity in erythrocytes were investigated for 37 weeks post-infection (pi). Cysts appeared in the mice at week 6 pi in the liver, after which they were found in the intestines and intestinal mesenteries and peritoneal wall, in addition to the liver. At week 11 pi the liver and spleen weights declined. The declination continued until week 33 pi, where they retained their' normal weights in comparison to the control, despite the fact that the spleen weight remained lower than that of the control. The mitotic index (MI) of bone marrow cells decreased significantly at week 26-33 pi and then recovered at week 37 pi. However, the decrease in the MI of the spleen cells was observed after the 4th week pi until week 37 pi. The decrease in the MI of the spleen cells coincided with the appearance of the cysts. Inhibition of CCP was observed in both bone marrow and spleen cells from week 2 till week 14 pi in the bone marrow cells and week 23 pi in the spleen cells , after which it became comparable to the control. This could possibly indicate that, the parasite has a cylotoxic effect on the immune cells. There was a significant increase in the frequency of sister chromatid exchanges (SCE) and chromosomal aberrations (CA) in the bone narrow cells but. riot in spleen cells. This suggests that the parasite may possess genotoxic factor(s) which affect bone marrow cells. The ADA specific activity was found to decrease from week 4 until week 29 pi, after which it became comparable to the control. The decrease in the: activity after week 4 pi coincided with the appearance of the. cysts. The return of ADA activity to normal at week 33 pi coincided with the return of liver and spleen weights to their normal level in comparison to the control. The reduction in liver and spleen weights, the reduction of MI, the inhibition of CCP and the reduction in ADA activity could be related to the immunosuppression state associated with the disease