Platelet-derived growth factor (PDGF) is the major mitogen in serum for connective tissue-derived cells. 1 , 2 Its in vivo function is not known, but its localization in the blood platelets in combination with the fact that PDGF acts on connective tissue cells has led to speculations that PDGF might act as a wound hormone. The finding that cytotrophoblasts of the early placenta both produce and respond to a PDGF-like growth factor 3 suggests that PDGF, acting via an autocrine mechanism, might also be of importance in the stimulation of placental growth in the early phases of development. Roles have also been ascribed to PDGF in certain pathological conditions involving cell proliferation, such as atherosclerosis, 4 6 myelofibrosis, 7 scleroderma, 8 and neoplasia. 9