The antitumor activity of delta 7-prostaglandin A1 (delta 7-PGA1) or delta 12-prostaglandin J2 (delta 12-PGJ2) on human ovarian cancer cell lines resistant to cisplatin (CDDP), doxorubicin (ADR), and L-phenylalanine mustard (l-PAM) was studied in vitro. A2780AD, A2780 (parent cells of A2780AD), 2008DDP, and 2008 cells (parent cells of 2008DDP) were used. The antitumor activities of the drugs were defined with 50% inhibitory concentration (IC50) estimated from growth inhibition curves, which were obtained by an indirect colorimetric method. Drug-resistance ratios obtained from IC50 values, by comparing A2780AD and A2780 cells, were 62.5 for ADR, 4.6 for CDDP, 4.9 for l-PAM, 1.5 for delta 7-PGA1, and 1.8 for delta 12-PGJ2. Those obtained by comparing 2008DDP and 2008 cells were 1.1 for ADR, 16.0 for CDDP, 2.9 for l-PAM, 2.3 for delta 7-PGA1, and 3.2 for delta 12-PGJ2. Thus some human ovarian cancer cells resistant to ADR, CDDP, and l-PAM remain sensitive to antitumor PGs.