Abstract
1. Human placental cord serum contains a factor or factors necessary for active growth of fibroblasts in tissue culture and a factor which prevents the appearance of a characteristic toxicity. 2. The growth-promoting factor(s) is stable to heating to 60 degrees C. for 30 minutes and is associated with the euglobulin fraction of the serum. 3. Extraction of dried cord serum with benzene, chloroform, and ether does not cause any decrease in growth-promoting activity. 4. Sera from hypophysectomized, adrenalectomized, and thyroid-parathyroidectomized rats show as much growth-promoting activity as normal rat sera. 5. Bovine gamma globulins, anterior pituitary growth hormone, and insulin do not replace the serum growth factor. 6. The toxicity-preventing factor is serum albumin. 7. Evidence is presented that the toxicity is due to free fatty acids in the medium and that serum albumin acts by binding the fatty acids.