A radioimmunoassay has been developed for the detection and quantification of a human serum polypeptide that has growth-promoting activity for confluent Balb/c-3T3 cells. Antiserum to this growth factor does not recognize antigens in mouse, guinea pig, or bovine serum but does detect some crossreacting antigen in the serum of the New World monkey Cebus albifrons and more in the serum of the Old World rhesus monkeys Macaca mulatta and M. fascicularis, demonstrating that the antigenic determinants of the growth factor have a degree of species specificity. Serum derived from whole human blood contains approximately 770 pg of the growth factor per mg of protein; serum derived from platelet-poor blood contains about 112 pg of the growth factor per mg of protein. As much as 1 microng of the growth factor per mg of protein has been recovered from human platelets by heating them at 100 degrees for 2 min. Approximately 1-2 ng of the growth factor, in either whole serum or platelets, stimulates 5 to 10 X 10(3) confluent Balb/c-3T3 cells to replicate. The heat treatment of platelets allows the purification and quantitative recovery of the growth factor from blood.