Growth inhibition or stimulation of target adenocarcinoma cells in rats sensitized with spleen cells from syngeneic tumor-bearing rats was significantly suppressed in colony inhibition assays when the spleen cells were treated in vitro with 0.8 mug progesterone or more/ml medium. In addition, when tumor-bearing rats were treated with 1 mg medroxyprogesterone acetate weekly in vivo, the inhibiting action of the spleen cells from rats with regressed tumors was also suppressed. Progesterone thus suppressed immune spleen cells in vivo and in vitro.