IL-I0 mRNA expression and protein production in established melanoma cell lines and freshly cultured primary and metastatic melanoma cells was examined. The in situ distribution of IL-I0 in native melanoma tissue was also investigated by immunohistochemistry in primary tumors, metastases, benign melanocytic nevi and normal skin of healthy persons and melanoma patients. IL-I0 mRNA, but not IL-I0 protein in the culture supernatant, was found in I of 4 cultured melanoma cells of primary tumors, while 3 of 6 melanoma-metastasisderived cultures expressed both IL-I0 mRNA and protein. No IL-I0 was detected in skin biopsies of healthy volunteers or in the healthy skin of melanoma patients; nor was IL-I0 found in congenital melanocytic nevi. In only I of the I I examined primary malignant melanomas was IL-I0 immunoreactivity detected within the cytoplasm of cells in the tumor. On the other hand, 4 of 9 metastases clearly displayed scattered IL-I O+ cells. In all sections with IL-10-positive cells, the cells were positive for HMB-45. No co-expression of CD3 and IL-I0 was observed. The data suggest that melanoma cells themselves are the main origin of IL-I0 in tumor specimens in vivo. The preferential expression of IL-I0 in metastatic lesions and in cultured cells from metastases might indicate an increased spreading potential of IL-10-secreting melanoma-cell clones.