Effects of dexamethasone on the ability of nonadherent bone marrow cells to inhibit proliferation of mastocytoma P815 cells and concanavalin A-stimulated proliferation of syngeneic splenocytes were studied. Antitumor activity of cells increased, but their natural suppressor activity decreased in the presence of dexamethasone. Pretreatment with dexamethasone (for 3 h) did not affect the sensitivity of mastocytoma cells to antitumor factors and the antitumor activity of bone marrow effectors. Pretreatment of bone marrow cells with dexamethasone for 24 h potentiated antitumor activity of their nonadherent fractions.