The different platelet-derived growth factor (PDGF) isoforms cause activation of their α and β protein tyrosine kinase receptors through dimerization. Homodimerization as well as heterodimerization of receptors occur. It has been shown previously that the heterodimeric receptor complex mediates a stronger mitogenic response than either of the homodimeric complexes. In this report, we show that in cells expressing both PDGF α-and β-receptors, stimulation with PDGF-AB, which leads to preferential heterodimer formation, leads to a very low degree of phosphorylation of Tyr771 in the β-receptor. In contrast, Tyr771 is phosphorylated in a homodimeric complex of β-receptors. Phosphorylated Tyr771 is a binding site for RasGAP; an analogous site is not present in the α-receptor, which lacks the ability to associate with RasGAP. The lowered phosphorylation of Tyr771 in the heterodimeric receptor complex correlates with lowered association with RasGAP, as well as with a more efficient activation of Ras and MAP kinase, which is consistent with the increased mitogenicity elicited by PDGF-AB, compared to PDGF-AA or PDGF-BB.