Lymphocyte subpopulations expressing the CD4+ and CD8+ antigen were analyzed in spleen, blood and caecal tonsil of SC and TK chickens infected with E.tenella. Following primary and secondary infections with E.tenella, SC and TK chickens were showed different levels of susceptibility to coccidiosis and showed different patterns of changes in T lymphocyte subpopulations. Highest level of CD8+ T lymphocytes was seen in SC birds compared with TK chickens following challenge infection. SC and TK chickens showed an increase in NO2 production 6 day post primary infection. TK chickens also showed an increase in NO2 production 12 and 27 day post primary infection. No difference in oocyst output between SC and TK birds was observed following primary infection with E.tenella. Following secondary infection, TK chickens showed significantly higher level of oocyst production 4.8xl06 /bird while SC birds showed 2.4 xlO6 /bird post secondary infection. Abbreviations FCM, flowcytometric analysis; C.T, caecal tonsil. mAb, monoclonal antibody.