In this study, the individual and combined effects of prostaglandin E1 (PGE1) and Indomethacin on mitomycin C (MMC)-induced SCEs in human lymphocytes was investigated in vitro. All MMC-treated cultures showed a great increase of SCEs (approximately two-fold), indicating its ability to induce mutations. SCE data showed that MMC-induced SCEs were reduced significantly in the presence of PGE1 in pooled analysis of six experiments (60.55% reduction of SCEs at 10(-6) M, 34.13% reduction of SCEs at 10(-9) M). In contrast the presence of indomethacin in the medium during MMC treatment of cells failed to show a significant reduction of SCEs in pooled analysis (21.17%). However, individual analyses revealed only two of six donors with a significant SCE response. Thus, the findings suggest that PGE1 can modify the DNA damaging effect of carcinogens and thereby may prevent the initiation of the carcinogenic process.