Cells of clone B(5)59, a derivative of the murine B16 melanoma, are highly tumorigenic, and produce both melanin and plasminogen activator. Cells of clone C(3)471, a line obtained by continued growth and maintenance of B(5)59 cells in medium containing 5-bromodeoxyuridine are nontumorigenic, amelanotic, and have no plasminogen activator. Differences in the mRNA complement of these two syngenic mouse melanoma clones have been determined by hybridization kinetics between complementary DNA transcribed from mRNA of either B(5)59 OR C(3)471 cells, and mRNA isolated from both sources. To detect the presence of unique mRNA sequences produced in B(5)59 cells, complementary DNA produced using B(5)59 mRNA as a template was exhaustively hybridized to C(3)471 mRNA. The results indicate that (i) polyadenylated mRNA produced by either clone can be divided into three main groups based upon the relative complexity of mRNA species within each group, (ii) more than 98% of the mRNA species produced by the B(5)59 cells are also produced by the cells of the C(3)471 clone, (iii) approximately 25% of the mRNA species produced by the cells of the C(3)471 clone in moderate abundance (500 molecules per cell) are not produced by the B(5)59 clone. Despite the fact that at least two of the proteins synthesized by B(5)59 cells are not detectable in C(3)471 cells, our results support the hypothesis that the major effect of BrdUrd incorporation into DNA is the induction rather than the repression of transcription of polyadenylated mRNA sequences.