The in vitro interaction between the new antimetabolite gemcitabine (GEM) and topotecan (TPT) was analyzed in A2780 ovarian cancer cells. The growth inhibitory effect was assessed after 3 days of drug exposure. GEM and TPT obtained in vitro IC50 values of 2.1 +/- 0.9 and 33.7 +/- 10.2 nM, respectively. The interaction between GEM and TPT was evaluated by exposing cancer cells at increasing doses of GEM (0.1, 1, and 10 nM) and TPT (1, 10, 100, and 1000 nM). Analysis of data about the interaction between GEM and TPT was performed by applying the isobole method. An antagonistic effect was noticed when GEM was combined with TPT in the tested concentration range. DNA analysis was also performed and showed an augmentation of cells blocked in the G2/M phase during TPT exposure, while an increase of blocked cells in the G0/1, phase was observed after GEM treatment. This latter effect was predominant when the two drugs were used in combination. We also investigated the effect of sequential exposure to drugs, pretreating A2780 cells for 24 h with TPT and then for 48 h with GEM, and, conversely, pretreating A2780 cells with GEM for 24 h and thereafter with TPT for 48 h. Both these combined sequential treatments showed an antagonist effect of the drugs' combination. Long-term growth inhibition effect was established by clonogenic assay performed after 10 days of culture after drug treatment. Also these data confirmed the antagonistic effect between GEM and TPT in A2780 ovarian cancer cells.