Transplantation of human tumors into the organ or tissue of their origin (orthotopic transplantation) in nude mice can result in significant enhancement of tumor growth and metastases, compared with se (ectopic) transplantation. Because melanocytes are normally found in the epidermal-dermal junction, intradermal inoculation of melanoma cells might be expected to improve their potential for malignant growth as xenografts. The purpose of our study was to examine this possibility. We found that because mouse epidermis and dermis are so thin, it was not possible to inject a bolus of tumor cells intradermally; instead the cells were actually deposited in the most superficial layer of the subcutis (i.e., subdermally). We evaluated the behavior of cells from a human melanoma cell line after sc or subdennal inoculation into National Institutes of Health Swiss athymic nude mice. The cells used were from (2) the predominantly amelanotic human malignant melanoma cell line MeWo, originally established from a melanotic lymph node metastasis, and (2) two Me Wo variants resistant to wheat germ agglu-tinin (WGA^r), which were selected for altered malignant capacities. Whereas 5×10⁵ MeWo cells were required to achieve 100% tumor take with sc injection, only 2×10⁴ cells were required with subdermal inoculation. Subder-mal injection of the MeWo cell resulted in the development of highly melanotic and nonencapsulated primary tumors, which grew quickly into the dermis and epidermis and metas-tasized at high frequency to draining lymph nodes. In contrast, the tumors that developed after sc injection were found in the deepest layer of the subcutis and were predominantly amelanotic and encapsulated; they rarely metastasized to lymph nodes. Neither of the WGA^r variants, called 70-W and 3S5, displayed such drastic changes in growth behavior after subdermal inoculation. These results suggest that the subdermis, although not a perfect orthotopic site for a melanoma implant, may be more favorable and appropriate than the deep subcutis. In some cases, the subdermis may facilitate or induce the expression of phenotypic traits characteristic of human malignant melanomas in their natural host, and it may enhance the success rates of transplantation of such tumors in nude mice. [J Natl Cancer Inst 81:938-944, 1989]