In primary cultures of rat preadipocytes (PA) isolated from epididymal or perirenal depots, rat serum is more effective than other animal sera (fetal calf, newborn calf, human, horse, rabbit, cat, sheep, goat, dog, pig) in promoting adipogenic conversion, biochemical differentiation, and mitogenesis. Only mouse serum is comparable to rat serum. This activity is attributable to a specific growth factor (preadipocyte stimulating factor, PSF). An assay for PSF in rat serum was devised using PA from perirenal fat of 3-month-old Fischer 344 rats grown first to confluence in FCS for 8 days and then for the next 3 days in test serum, followed by measurement of triglyceride (TG) and glycerol-3-phosphate dehydrogenase (GPDH). Rat serum induces dose-dependent rapid cell division, which coincides with accumulation of TG and increase of GPDH; for routine quantitation, TG is assayed. The biochemical characteristics of PSF in serum are as follows: stable at 4 degrees C for up to 1 year; inactivated at 100 degrees C (80% loss, 30 min) but stable at 56 degrees C for 1 hr; stable at pH 2-12; non-dialyzable; completely resistant to pepsin, trypsin, and chymotrypsin but destroyed by pronase and subtilisn; stable to DTT and periodate; and m.w. between 68 kDa (Sephacryl-300) and 58 kDa (Sephacryl-300 in 5 M urea). PSF activity is greater in serum from Wistar than from Fischer 344 rats, while activity of serum from Zucker obese (fa/fa) rats is at least as great as that from Wistar rats and, like serum of rats made obese by feeding a high-fat, high-carbohydrate diet, is not suppressed. PSF activity is not due to insulin, insulin-like growth factor-1 (IGF-1), growth hormone, glucocorticoids, or combinations of these hormones. PSF activity was not seen with a number of growth factors including colony-stimulating factor (CSF-1), GM-CSF, interleukins 1, 2, and 3, neuroleukin, tumor necrosis factor, and others. PSF is distinct from the low molecular weight (4-8 kDa) differentiation factor present in rat serum, FCS, and human serum that promotes the adipogenic conversion and cellular differentiation of 3T3-L1, 3T3-F442A, and Ob17 cells. PSF appears to be a new differentiation factor for rat preadipocytes, has properties suggestive of a highly glycosylated protein, and may be highly species specific.