Morphological examination of HeLa cells exposed to etoposide for 1 h revealed two distinct modes of death: (a) within 6 h of drug removal, shrunken cells appeared which contained vacuolated cytoplasm and regions of intense chromatin staining, consistent with apoptosis; and (b) concomitant with release from G₂ arrest, enlarged cells appeared which contained evenly staining nuclear fragments, consistent with mitotic death. The methylxanthine, caffeine, enhanced cytotoxicity in a concentration-dependent manner when applied for 24 h following etoposide exposure. One mm caffeine alleviated etoposide-induced G₂ arrest and increased the incidence of mitotic death, accounting for the potentiation of cytotoxicity. Brief caffeine exposures (5 or 10 mm for 1–2 h) caused specific tyrosine dephosphorylation and activation of p34^cdc2 kinase, and mitotic progression to a limited extent, in cells which were arrested in G₂ following etoposide treatment. However, longer exposure times at a high caffeine concentration (10 mm) caused inhibition of both cell cycle progression and mitotic death, and the enhancement of etoposide cytotoxicity could be accounted for by up to a 3-fold increase in the proportion of morphologically apoptotic cells. Thus, caffeine potentiates etoposide cytotoxicity by two morphologically distinct mechanisms depending on its concentration.