Tumor growth and changes in T and B lymphocyte ratio in spleen, draining lymph node and peripheral blood of thymectomized, irradiated rats, reconstituted with syngeneic bone marrow transplanted at various time intervals with MC-1 fibrosarcoma cells were followed. Control nonthymectomized or "sham" operated rats were transplanted an equal dose of tumor cells. Thymectomy and irradiation reduced the numbers of T lymphocytes in all lymphoid organs, while the enhanced numbers of B cells are probably related to reconstitution with cells of syngeneic bone marrow. The time interval between thymectomy, irradiation and transplantation of tumor cells proved to be a limiting factor for tumor growth and changes in T and B cell ratio. Early transplantation of tumor cells (7 days after irradiation) resulted in an enhanced resistance to tumor development, a reduced tumor growth rate and a progressing decline in the number of T cells. If the interval between thymectomy and tumor cell transplantation lasted 4 weeks, the T cell population became partially regenerated, and tumors grew progressively in correlation with a continuing T lymphocyte depletion. The results are discussed in terms of the role of various T cell subpopulations and the significance of residual, thymectomy- and irradiation-resistant T lymphocyte population, vital for a preservation of T cell immunological functions.