We have studied the effects of medroxyprogesterone acetate (MPA) on C6 glioma growth in vitro in order to prove the hypothesis that it could arrest growth and induce drug sensitisation in a glial tumour as it does in breast cancer cells. Plating, thymidine-labelling index, ultra-structure, and soft agar colony growth were determined after incubation with MPA, and/or cisplatin, procarbazine and methotrexate (MTX). MPA (1 mug/ml) reduced the thymidine-labelling index by 41 and 73% at 48 and 96 h, respectively, and decreased colon growth by 61%. Soft agar colony inhibition by NIPA was almost as potent as MTX (0.3 mug/ml), but the latter drug showed very high cytotoxicity. Electron ri-i-icroscopy revealed that in medroxyprogesterone treated cells myeloid bodies developed, but MTX treatment caused mainly necrosis. Medroxyprogesterone increased procarbazine and cisplatin-induced colony growth and S-phase inhibition, but reduced MTX-induced thymidine-labelling inhibition. In conclusion, progesterone may inhibit growth and sensitize to drugs. (C) 2001 ISDN. Elsevier Science Ltd. All rights reserved.