Four new human melanoma cell lines were established in monolayer culture from xenograft lines originating from different patients. Several distinct characteristics of the source xenograft lines were retained in the cell lines, e.g., number of chromosomes, DNA-index, and cell ultrastructure. Cell volume was generally larger for the cell lines than for the corresponding xenograft lines, but the differences among the lines were similar in vitro and in vivo. The cell lines showed significant differences in growth pattern, i.e., cell motility and degree of intercellular contact. Cell cycle time (Tc) during exponential growth ranged from 15 to 21 h. The differences among the lines in Tc were mainly due to differences in the duration of S. Growth fraction was close to 100% and cell loss was negligible during exponential growth. Plating efficiency was 90-100% in the presence of feeder cells. The four cell lines represent a valuable supplement to the xenograft lines for future studies of the cell biology, pathophysiology, metastatic behavior, and treatment sensitivity of malignant melanoma.