Tumor necrosis factor alpha (TNF alpha) is a cytokine, produced by lymphocytes and monocytes, with cytotoxic activity against some but not all tumor cell lines. Resistance to the cytolytic effects of TNF alpha has been reported in cell lines with autocrine TNF alpha production. The purpose of this study was to investigate whether human primary malignant melanoma and tumor infiltrating lymphocytes produce TNF alpha in vivo. Optimal conditions for in situ hybridization for TNF alpha mRNA in paraffin-embedded tissue were established. Analysis of 13 primary malignant melanomas and 3 metastatic lesions with different degrees of immunohistochemical TNF alpha positivity demonstrated that, in some tumors, both melanoma cells and leukocytes contained TNF alpha mRNA and protein. These findings demonstrate variable production of TNF alpha in primary and metastatic melanoma in vivo. The previously described resistance to TNF alpha cytolytic activity may, therefore, be clinically important.