Cell-mediated cytolysis (CMC) was assayed in a system using spleen cells from mice (C57BL/6) sensitized with allogeneic tumour cells (DBA/2 mastocytoma P-815). Anti-inflammatory drugs, immunosuppressives, inhibitors of cell division and other agents were investigated for their capacity to inhibit CMC in three different ways. First, inhibition of CMC after in vitro addition of drug was observed with corticosteroids, some immunosuppressives and inhibitors of cell division. Secondly, suppression of CMC after a single drug administration to sensitized mice shortly before being killed was found with corticosteroids, several immunosuppressives and irradiation. Thirdly, prevention of development of CMC by repeated drug treatment (immunosuppressive schedule) was achieved with most immunosuppressives and cytostatic drugs. Non-steroidal anti-inflammatory drugs were inactive in these tests. Correlation of effects between the three procedures was very poor and it is suggested that various mechanisms may be involved in the different assays.