Competition between 125I-labeled platelet-derived growth factor (PDGF) and unlabeled PDGF forms the basis of a specific "radio-receptor assay" for quantifying PDGF in clotted blood serum. Human clotted blood serum contains 15 ng/ml of PDGF by radio-receptor assay; this corresponds to a PDGF content of approximately 7.5 x 10(-5) pg per circulating platelet, a figure which is corroborated by purification data. Clotted blood sera from mammals, lower vertebrates and marine invertebrates were screened for homologues of human PDGF by radio-receptor assay. All tested specimens from phylum Chordata contain a mitogenic agent that competes with human PDGF for receptor binding. Sera from tunicates down on the chordate line of evolution and sera from all tested animals on the arthropod line of development were negative. The phylogenetic distribution of PDGF homologue does not correlate with platelet distribution since platelets and their precursor cell--the bone marrow megacaryocyte--are unique to the mammalian hematopoietic system. One anatomical feature appearing coordinately with PDGF on the vertebrate line of development is a pressurized circulatory system. The coincidental appearance of these features may lend support to the hypothesis that PDGF plays a role in maintenance and repair of the vascular lining in vivo.