Several isoforms of the platelet-derived growth factor (PDGF) have been identified and purified to date. The functional effects of the PDGF isoforms differ, due to their different interactions with at least two distinct cell surface receptors. Thus, PDGF constitutes an example of an expanding growth factor family, characterized by possibilities for fine-tuned regulation of action. In this manuscript, we describe the structural features of the PDGF isoforms as well as their receptors.