The action of high-dose medroxyprogesterone acetate (MPA) was studied by analysing the behaviour of colony-forming-unit granulocyte-macrophage (CFU-GM) during chemotherapy. 21 non-pretreated men with locally advanced carcinoma of the head and neck were randomised into two arms: A (11 patients) received three alternating cycles of cisplatin, 5-fluorouracil (CF)/cisplatin, methotrexate, bleomycin, vincristine and then CF every 4 weeks and B (10 patients) were treated with the same schedule plus 1000 mg per day of MPA. MPA was administered 14 days before the start of chemotherapy (day 0) and continued daily up to the 90th day. Bone marrow was harvested in arm A on days 0, +14 and +90, and in B, also on day -14. There was diverse CFU-GM behaviour in the two arms on the 14th day. These data support the hypothesis that the myeloprotective effect of MPA is due to induction of a mitotic rest in the stem cells, which protects them from drug action.