Quantitation of T cell subsets in intraepithelium and lamina propria during the course of experimental G. lamblia infection in the inbred mice revealed increased influx of Thy 1.2+ (T cells) and Lyt 2.2+ (suppressor/cytotoxic T cells) during the establishment (3-5 day post-inoculation) and acute (9-11 day post-inoculation) phases of infection. The influx of these cells reduced as the parasite load declined. In contrast, no significant changes were noticed in lamina propria and intraepithelium Lyt 1.1+ (helper T cell) cells during the establishment or acute phase but such cells increased significantly in the decline (17-21 day post-inoculation) phase of infection. Further, both intraepithelium and lamina propria lymphocytes isolated from uninfected or infected animals failed to kill G. lamblia trophozoites in vitro in the absence or presence of antigiardial antibodies. Our data suggest that the clearance of G. lamblia trophozoites was not mediated by cytotoxic T cells. However, the induction of helper T cells during the declining phase of infection might be an important mechanism for the induction of parasite specific antibody response leading to the immune elimination of G. lamblia trophozoites from the gut.