The dynamics of changes was followed in the migratory properties of T lymphocytes from lymphoid organs of tumor-bearing rats, labeled in vitro with 3H-uridine and injected i.v. to normal recipients, a comparison being made with the migration of T cells from normal donors in syngeneic recipients. The progressive tumor growth was found to correlate with a decline of the ability of spleen T cells from a tumor-bearing rat to migrate to the spleen and lymph nodes of normal animals, following a transient striking migration in the early stages after inoculation with tumor cells. The site of an intensive migration was the liver. T lymphocytes of lymph node draining tumor migrated in correlation with its growth in an enhanced percentage into the spleen, and in a reduced ratio to the lymph node of a normal recipient. T lymphocyte migration of all the lymphoid organs followed became normalized in the stage of the tumor rejection. Peripheral blood T lymphocytes from a tumor-bearing donor showed a depressed ability to migrate into normal lymphoid organs, while the migration rate of thymocytes from such donors in normal recipients underwent no change with the exception of an increased migration into the liver during tumor regression. The results showed changes in the migratory properties of labeled T cells to depend on the immune status of the donor and the lymphoid organ source of the injected lymphocytes.