Members of the epidermal growth factor (EGF) family of structurally related tyrosine kinase receptors, known as the ErbB receptors (EGFR/ErbB1/HER1, ErbB2/HER2/neu, ErbB3/HER3 and ErbB4/HER4) and their respective ligands, have been suggested to be involved in the development and progression of malignant melanoma. Here we investigate the effects of the ErbB1 tyrosine kinase inhibitor gefitinib (ZD1839, Iressa) on human malignant melanoma cells (RaH3 and RaH5) in vitro. ZD1839 inhibited proliferation of exponentially growing RaH3 and RaH5 cells in a dose-dependent manner with a half-maximally effective dose of 3.5 and 2.0 micromol/l, respectively. Cell growth was inhibited at 0.1 micromol/l ZD1839 in both cell lines. Maximal inhibition was accomplished at 10 micromol/l ZD1839; however, the effect was not complete as both cell lines showed a continuous slow growth during the treatment period. Flow cytometry analysis of cell-cycle distribution showed that ZD1839 treatment caused accumulation of RaH3 and RaH5 cells in the G1 phase. The growth arrest induced by ZD1839 coincided with upregulation of the cyclin-dependent kinase inhibitor p27 KIP 1. There was no increase in apoptosis as determined by analysis of plasma phosphatidyl serine redistribution. Western blot analysis revealed that ZD1839 substantially reduced tyrosine phosphorylation of ErbB1 as well as ErbB2 and ErbB3. This was accompanied by a concomitant decrease in Akt-phosphorylation, Erk1/2-phosphorylation, and Stat3-phosphorylation. Our results show that ZD1839 interferes with the growth of human malignant melanoma cells by cytostatic effects. These findings indicate the possible use of ErbB receptor kinase inhibitors as a novel treatment strategy in malignant melanoma.