Pterin-dependent tyrosine hydroxylase has been described to occur occasionally in melanocytes. It is therefore important to quantify the mRNA of this enzyme in pigment cells to understand whether this enzyme can take an active part in pigment formation. A real-time reverse transcription-polymerase chain reaction method was used to quantify tyrosine hydroxylase mRNA in melanocytes and melanoma cells. The calibrator was obtained by amplification of a segment of cDNA from tyrosine hydroxylase mRNA, which included the target thus allowing enumeration of the number of transcripts per cell. In melanocytes (n = 3), tyrosine hydroxylase mRNA ranged from non-detectable to 0.000492 transcripts/cell and in melanoma cells from non-detectable to 0.005340 transcripts/cell. In neuroblastoma cells, the median tyrosine hydroxylase mRNA number was 0.4 transcripts/cell (range 0.02-25 transcripts/cell). The amount of tyrosine hydroxylase mRNA in the pigment cells was far less than the mRNA concentrations of four melanocyte-specific proteins measured in the same melanocytes and melanoma cells. We conclude that on the average less than 1 of 1000 melanocytes and melanoma cells contains at least one tyrosine hydroxylase mRNA molecule. Consequently, in 999 of 1000 cells translation into the corresponding enzyme protein cannot occur because of the lack of an mRNA template. Thus, in these cells there is no pterin-dependent tyrosine hydroxylase that can contribute to pigment formation by producing priming amounts of l-dopa for proper function of tyrosinase.