Human platelet-derived growth factor (PDGF) consists of two distinct but related polypeptide chains designated PDGF-A and PDGF-B. The gene encoding PDGF-B has given rise to the v-sis oncogene. In the present study the transforming activities of PDGF-A and PDGF-B genes are compared. The PDGF-A chain gene is markedly less efficient in inducing transformation than the PDGF-B gene under the influence of the same promoter. There are significant differences in the secretory and growth stimulating properties of the two chains. These properties appear to account for the much more potent transforming ability of the PDGF-B gene. These findings provide insights into biologic properties of a growth factor responsible for potent autocrine stimulation of abnormal cell proliferation.