The characteristics of 7 human melanoma cell lines were compared with those of the xenografts from which they were established. The ultrastructure, melanin content, isozyme pattern and chromosome numbers of the cell lines were closely similar to those of the corresponding xenografts. The different cell lines gave rise to colonies in soft agar of size and morphology similar to the parent xenografts, and the plating efficiencies were clearly correlated. However, no correlation was found between the growth rates in vivo and either the doubling times and saturation densities in monolayer cultures or the plating efficiencies in soft agar. Moreover, one of the cell lines lost its tumorigenic ability upon establishment in culture. Thus, although the properties of the cell lines by and large reflected those of the parent xenografts, important inconsistencies were seen. The data emphasize that extrapolations from continuous cell lines in vitro to tumour cells in vivo are not necessarily valid. A high content of cellular fibronectin was correlated with a compact colony morphology in soft agar and rapid attachment and spreading on plastic. The growth rates and cellular morphology of the cell lines were strongly influenced by TPA, DMSO, retinoic acid and theophylline, but not by alpha-melanocyte-stimulating hormone. A murine cell line established from one of the xenografts grew in soft agar and produced sarcoma in mice. The malignant murine cells had arisen by transformation of murine stromal cells during the first subcultures in vitro, possibly caused by a factor produced by the human melanoma cells.