Both humoral and cell-mediated immune responses were studied in reactivated toxoplasma infection in mice. The infection was established by immunosuppressing chronically infected animals by thymectomy followed by irradiation. The reactivated infection was confirmed by reappearance of toxoplasma tachyzoites in peritoneal exudate. The animals survived until the 8th day post thymectomy. The percentage of splenic T lymphocytes was depressed after 4th day post thymectomy in both the uninfected thymectomised and infected immunosuppressed animals as compared to the intact uninfected controls and intact chronically infected animals. However, there was no significant change (P > 0.05) between the uninfected immunosuppressed control and infected immunosuppressed mice. When the infection progressed to more acute phase there was significant depression (P< 0.01) in the percentage of T lymphocytes in comparison to the uninfected thymectomised mice. However, there was no change in the percentage of B lymphocytes (P> 0.05). The function of lymphocytes as assessed by blast transformation was depressed in the presence of nonspecific mitogens, i. e. phytohemagglutinin and concanavalin A until the end of the study. The response to toxoplasma antigen was increased during early stages in the infected immunosuppressed mice (stimulation index = 4.15 ___ 0.1 and 4.5 ___ 0.15 on 4th and 6th day post thymectomy ). On the 8th day post thymectomy, there was a sudden fall in the stimulation index (3.5 + 0.11). The antitoxoplasma antibodies as detected by indirect hemagglitinin and fluorescence analysis and enzyme-linked immunosorbent assay (ELISA) tests and toxoplasma-specific IgM antibodies as detected by ELISA test were present throughout the study period though at low levels. The significance of these findings is discussed.