Most studies of the reverse transcriptase in situ polymerase chain reaction technique have reported results from assessments of cultured cells, frozen sections, and cytospin preparations. For application to routine diagnosis, it will be necessary to adapt the technique for use with formalin-fixed, paraffinembedded tissues, the materials that are generally available. We have evaluated the feasibility of such an approach, using surgical pathology archival material from 25 UCLA patients: 15 tissues from primary and metastatic melanoma, 7 from non-melanocytic tumors, including cancer of the lung, colon, kidney and skin and a thyroid adenoma, and 3 nontumorous tissues. Seven of 15 melanoma tissues gave a strong positive signal, 5 gave a weak signal, and 3 were negative. None of the 10 non-melanoma tissues gave a positive signal. The specific reaction product was mainly located in the cytoplasm. None of the nonmelanocytic tumors or normal tissues demonstrated this pattern of cytoplasmic staining. Some nonspecific nuclear staining was observed in melanocytic and nonmelanocytic tumors and must not be overread as a true positive result. It is possible to detect tyrosinase mRNA in formalin-fixed, paraffinembedded tissue sections of melanoma, but the technique remains too demanding for routine application.