We have compared the effects of a broad range of clinically relevant concentrations (0.1 to 10 M) of the steroidal pure anti-estrogen ICI 182,780 and the non-steroidal partial antiestrogen tamoxifen (TAM) on cell proliferation and induction of apoptosis in the estrogen receptor (ER)-negative ovarian carcinoma cell line A2780. Cell proliferation was assessed by evaluating the number of viable cells, changes in cell-cycle distribution and cell replication rate; while apoptosis induction was assessed by examining nuclear morphological changes associated with apoptotic death and DNA cleavage into 300 and 50 kbp units (large DNA fragmentation) and into 180 bp units (internucleosomal DNA fragmentation). We provide evidence that 0.1 to 10 M ICI 182,780 and TAM significantly inhibit the growth of A2780 cells in a dose-dependent fashion. Cytokinetic analysis revealed that only 10 M TAM caused a significant blockade in G 1 and a diminished replication rate. Conversely, we show that 0.1 to 10 M ICI 182,780 and TAM induce apoptosis in a dose-dependent fashion. The earliest recognizable apoptotic change induced by treating the cells with these 2 drugs was DNA cleavage into 300 and 50 kbp units. This started to be visible in adherent cells, implying that apoptosis induction by ICI 182,780 and TAM was not determined by the loss of cell-substrate interaction. A further degradation of 300 and 50 kbp DNA fragments occurred in cells that had lost their adhesion to the culture plate. We observed the ladder pattern typical of internucleosomal DNA cleavage by treating A2780 cells with the highest dose (10 M) of ICI 182,780 and TAM. Lower concentrations of these 2 drugs (0.1 to 1 M) did not produce such a pattern of DNA fragmentation. Typical features of apoptotic nuclei were detectable after both drug treatments. However, cells undergoing apoptosis induced by ICI 182,780 showed hyperaggregation of chromatin, whereas TAM-treated cells preferentially exhibited chromatin clumping.