Using site-directed mutagenesis of a PDGF-A cDNA clone, we identify two domains that are required to generate stable, mitogenically active PDGF-AA homodimers. Alteration of the tetra-basic amino acid sequence (Arg84-Arg-Lys-Arg to Arg-Ser-Asn-Gly) results in the formation of stable pro-PDGF-A homodimers that lack mitogenic activity. Substitution of serine for Cys129 destabilizes PDGF-A subunits within the cell. Genes incorporating either the processing lesion or the cysteine substitution suppress wild-type PDGF-A gene expression in a trans-dominant fashion. Suppression occurs because the mutant PDGF subunits dimerize with wild-type subunits to form inactive or unstable heterodimers. Suppression is exerted across phylogenetic boundaries; thus, the mouse PDGF-A chain mutants inhibit the activity of the wild-type Xenopus PDGF-A. The cysteine mutant gene suppresses expression of PDGF-B (c-sis), as well as PDGF-A. The processing mutant gene, however, suppresses only PDGF-A. Dominant-negative mutations of PDGF and other growth factors which, like PDGF, function as dimers may prove useful for creating animals models of growth factor deficiency disease states and for revealing the function of growth factors during early embryonic development.