Six new human melanoma xenograft lines differing considerably in biological properties were established in athymic nude mice and characterized with respect to growth in vivo. Mean volume-doubling time ranged from 1.8 to 17.7 days, mean necrotic fraction from 7 to 39%, and mean fraction of cells in S from 11 to 31% at a tumor volume of 500 mm3. DNA-index and median chromosome number were in the ranges 1.15-1.76 and 46-80, respectively. The lines were all highly malignant by histopathological criteria, but showed significant differences with respect to cell type, cellular pleomorphism, nuclear atypia, nucleolar prominence, and mitotic activity. Several biological characteristics of the donor patients' melanomas were retained in the xenograft lines. All lines showed a human karyotype and the histological appearance was very similar to that of the source tumor material. The distribution of cells in the cell cycle was comparable to that reported for melanomas in man. The six melanoma xenograft lines constitute a tumor panel that shows great promise for future studies of the biology and treatment sensitivity of malignant melanoma.