Microsporidia are protozoan parasites that cause opportunistic infections in persons with AIDS and also infect a wide range of hosts including laboratory animals (mice, rabbits, dogs, and monkeys). Murine models are being used for studying the immune responses against microsporidia and previous studies have shown that resistance is dependant on functional T lymphocyte responses. Athymic mice die from infection with many species of microsporidia and resistance can be transferred with sensitized syngeneic splenic T cells. Furthermore, macrophages can be activated to inhibit growth of or kill microsporidia in the presence of tumor necrosis factor (TNF `) or interferon gamma (IFN ), respectively. This macrophage mediated killing was found to involve the production of nitrogen intermediates. In the present study, splenic lymphocytes were collected from BALB/c mice various times after intraperitoneal inoculation with Encephalitozoon intestinalis and incubated in vitro with Enc. intestinalis for 24 hr. The supernatants were then tested for the presence of cytokines. Levels of IFN and interleukin 2 were significantly higher in supernatants of spleen cells from the E. intestinalis inoculated mice collected 7, 10 or 21 days after inoculation than in the cultures of spleen cells from uninoculated mice. Levels of interleukin 10, however, were not significantly different from cultures containing spleen cells from inoculated mice vs uninoculated mice. This suggests that Th1 type responses are important in controlling E. intestinalis infections in mice. In additional blastogenesis studies, spleen cells from the E. intestinalis inoculated mice and uninoculated mice as controls, were incubated with parasite fractions to determine which proteins were immunogenic. Statistically significant blastogenesis responses were detected against proteins with molecular weights of 11, 23, 120, 133, and 165 kDa. Studies are continuing to determine which cytokines are induced by each of these antigen fractions. In addition, humoral responses (local and systemic) are being monitored against dominant parasite antigens and murine enterocyte cultures are being established to study pathogenicity of microsporidia.