A spontaneous large granular lymphocyte leukemia from a F344 rat was transplanted to 36 syngeneic recipients to study the interactions among leukemia, T lymphocytes, and the development of immunemediated hemolytic anemia. Six rats were euthanatized at biweekly intervals, and spleen weight, total spleen cellularity, and differential spleen cell counts were correlated with hemograms and osmotic fragility. Sequential changes in splenic architecture were correlated with hematologic parameters. Monoclonal antibodies defining all T lymphocytes (W3/13), T helper-inducer cells (W3/25), and T suppressor cells (OX-8) were used to identify T cells in immunocytochemical techniques on spleen sections, as well as in fluorescence activated cell sorter analysis of spleen cell suspensions. The onset of hemolytic anemic at 7 weeks after transplantation coincided with the first detection of tumor cells in the spleen and peripheral blood. Tumor cells first accumulated in the marginal zones, and then they infiltrated the red pulp sinusoids. Although the leukemia caused dispersion of the splenic lymphoid tissue, there was no significant lymphopenia, and the relative number of helper (W3/25+) and suppressor (OX-8+) lymphocytes did not change. Because the induction of anemia was a relatively early event in splenic involvement, we concluded that anemia was unrelated to disruption of lymphoid architecture; furthermore, it does not appear to be caused by changes in the numbers of regulatory T lymphocytes.