Six cell lines of human malignant melanoma: A375, A375.2, G361, HMV-1, MM8.1 and WM115 were seeded at densities of 1x10(4) cells/ml, 2 x 10(4) cells/ml or 3 x 10(4) cells/ml of RPMI medium supplemented with 10% fetal calf serum and antibiotics in a humidified atmosphere of 5% CO2 at 37 degrees C. A375 cells were also grown in Dulbecco's minimum Eagle's medium (DMEM medium). The morphology was studied by phase contrast light microscopy. At 4 days after seeding the colonies of A375 cells and HMV-1 cells were oval-shaped, the cells were polyhedrical and were making contact with each other regularly. The remaining cells were scattered, more elongated, and made contact randomly. G361 cells and MM8.1 cells tended to form superposed layers before 100% confluency was achieved. There were great differences in the growth rate and doubling time of melanoma cells. The doubling time in day 1 was short (around 6-12 h) in the case of A375, G361 and HMV-1 cells, longer (around 18 h) in the case of MM8.1 cells and very long (ranging between 26 and 89 h) for A375.2 and WM115 cells. There were also differences in the doubling time of cells as a function of the cell density at seeding. On the other hand, except for MM8.1 cells, there were differences between the doubling time in day 2 compared to day 1.