Three different melanocyte-specific mRNAs are studied as potential markers for circulating melanoma cells in the serum of mice inoculated subcutaneously with B16F10 melanoma cells. These three mRNAs encode tyrosinase, tyrosinase related protein-2 (TRP-2) and Pmel17, proteins that are essential for the synthesis of melanin and are expressed specifically in melanocytes. We used reverse-transcription polymerase chain reaction (RT-PCR) to detect these three different melanocyte-specific mRNAs in the sera of B16F10 bearing mice. Since melanocytes would not normally be present in the blood, the detection of those transcripts should indicate the presence of circulating melanoma cells. RT-PCR detection of all three mRNAs was highly sensitive and specific. Our in vitro studies show that as few as 10 melanoma cells can be detected in 125 microl blood and that in vivo, melanoma cells can be detected in blood samples from B16F10 melanoma bearing mice. Of these three mRNAs, Pmel17 mRNA is the most sensitive marker for detecting circulating melanoma cells compared with tyrosinase mRNA and TRP-2 mRNA. Moreover, this mouse model might be useful for basic research of malignant melanoma patients with haematogenous metastasis.