Two different melanocyte-specific mRNAs are studied as markers for circulating melanoma cells in vitro using the human melanoma cell line G361 and in vivo using blood samples from Japanese melanoma patients at different clinical stages. These mRNAs encode tyrosinase, the most essential enzyme for melanin synthesis, and gp100, a melanosomal matrix glycoprotein recognized by monoclonal antibody HMB-45. We used reverse-transcription polymerase chain reaction (RT-PCR) to detect tyrosinase mRNA and gp100 mRNA in peripheral blood. Since melanocytes would not normally be present in peripheral blood, the detection of those transcripts should indicate the presence of circulating melanoma cells. RT-PCR detection of these two mRNAs was highly sensitive and specific. Our in vitro study showed that as few as 10 melanoma cells in 0.125 ml normal blood could be detected. In in vivo study, 130 blood samples from 55 melanoma patients gave positive and variably sensitive results, whereas no samples from healthy controls or patients with other cancers gave positive results. Tyrosinase mRNA was not detected in any of the melanoma patients. gp100 mRNA was detected in 12 of 55 melanoma patients, in none of five stage I patients (0%), in four of 26 stage II patients (15.4%), in one of six stage III patients (16. 7%) and in seven of 18 stage IV patients (38.9%). Thus gp100 mRNA is a more sensitive marker for detecting circulating melanoma cells compared with tyrosinase mRNA.