We have studied a human malignant melanoma cell line (WM115) which expresses high levels of PDGFA and PDGFB mRNAs, a hamster fibroblast cell line (Wq3H) which lacks PDGFB and has a very low expression of PDGFA mRNA, and hybrids between these cell lines. The PDGFA and PDGFB mRNA expression in the hybrid cells was markedly suppressed, indicating that the melanoma PDGF genes are controlled in the hybrid cells by negative factors/mechanisms contributed by the fibroblast genome. The melanoma but not the fibroblast PDGFB mRNA was induced by cycloheximide in the hybrid cells, indicating that the fibroblast and the melanoma PDGFB genes are regulated differently despite their presence in the same intracellular environment.