S-100 immunoreactivity in melanomas of the South American opossum Monodelphis domestica.

S-100 immunoreactivity was determined 1) in foci of melanocytic hyperplasia, 2) in naturally occurring, ultraviolet radiation-induced, and 9,10-dimethyl-1,2-benzanthracene (DMBA)-induced primary melanomas, and 3) in metastatic melanoma lesions in the South American opossum Monodelphis domestica. Preneoplastic lesions of melanocytic hyperplasia contained scattered cells with S-100-positive nuclei. All primary melanomas, with the exception of a single DMBA-induced tumor, contained cells with S-100-positive nuclei. The pattern of S-100 reactivity in tumors varied from large foci of S-100-positive cells to scattered individual S-100-positive cells. Lymph node metastases were S-100 positive, but metastatic masses in internal organs were usually S-100 negative. Although S-100 reactivity did not distinguish preneoplastic lesions from tumors or benign melanomas from malignant melanomas, identification of metastatic tumor cells clearly demonstrated malignancy.