The induction of apoptosis by anticancer drugs and its relationship to stages of the cell cycle was studied in cells derived from a solid tumour; a highly malignant hamster fibrosarcoma (Met B). Asynchronously proliferating cells were treated with a wide variety of agents such as actinomycin-D, 1-beta-D-arabinofuranosyl cytosine, camptothecin, cisplatin, cyclophosphamide, daunorubicin, 5-flurouracil, 6-mercaptopurine, hydroxyurea, ionomycin, methotrexate and vincristine. With the exception of cyclophosphamide and hydroxyurea, a 36 h exposure to these drugs resulted in inhibition of cell growth and apart from cyclophosphamide, hydroxyurea. 6-mercaptopurine and cisplatin the induction of apoptosis. Studies using a decreased concentration of drug and exposure time (12 h) followed by examination of cells using flow cytometry indicated that most drugs were capable of affecting cell cycle progression without induction of apoptosis. However when cells were synchronised at G0/G1, S and G2/M phases and then exposed to these decreased concentrations of drug apart from 6MP an HU, apoptosis was observed and for the majority of drugs it took place in the same phase in which progression through the cell cycle was blocked by the drug. Cells synchronised in G0/G1 phase were more susceptible to methotrexate, whereas S-phase cells were more susceptible to camptothecin and 5-flurouracil and G2/M phase cells more susceptible to actinomycin D, 1-beta-D-arabinofuranosyl cytosine, daunorubicin and cisplatin. In contrast, vincristine blocked cells in G2/M phase but exerted its apoptotic effect in S-phase cells, ionomycin had no effect on the cell cycle, but G2/M cells appeared to be more susceptible to the effect of this drug. These data indicate that entry into apoptosis by this fibrosarcoma may occur at any point in the cell cycle. They also demonstrate a correlation between the action of some anticancer drugs on the cell cycle and the subsequent induction of apoptosis which may be useful in chemotherapeutic design.