To study the immune response to coccidiosis, the suckling rabbits were inoculated with 2,000 oocysts of either Eimeria intestinalis or Eimeria flavescens at 19, 22, 25, 29, and 33 days of age (DA) and in the case of E. intestinalis at 14 and 16 DA as well and sacrificed 14 days later. Another group served as an uninfected control and the rabbits were killed at the same age as their infected counterparts. Unlike the antibody response, the parameters reflecting cellular immunity (total number of leukocytes in mesenteric lymph nodes, lymphocyte proliferation upon stimulation with specific antigen and the dynamics of CD4+ and CD8+ cell proportions in the intestinal epithelium at the specific site of parasite development) were significantly changed from about 25 DA onwards. In contrast to the rabbits infected with weakly immunogenic coccidium E. flavescens, the proportions of CD4+ and CD8+ lymphocytes in intraepithelial lymphocytes from the specific site of parasite development were considerably changed after infection with highly immunogenic species E. intestinalis. As the immune system of sucklings from about 25 DA reacts to the infection, this age may be considered in terms of vaccination against coccidiosis.