Platelet-derived growth factor (PDGF) is a mitogen for connective tissue cells and occurs as disulphide-bonded homodimers or heterodimers of related polypeptide chains. Recent data indicate that the isoforms have different functional activities due to the fact that they bind with different affinities to two distinct receptor types. The frequent expression of PDGF and PDGF receptors in normal as well as transformed cells, suggests roles for PDGF in autocrine and paracrine stimulation of cell growth in vivo.