The effect of human epidermal growth factor (hEGF), a 5,400 molecular weight polypeptide isolated from human urine, on the growth of human foreskin fibroblasts (HF cells) was studied by measuring cell numbers and the incorporation of labeled thymidine. The addition of hEGF to HF cells growing in a medium containing 10% calf serum resulted in a 4-fold increase in the final density. The presence of hEGF also promoted the growth of HF cells in media containing either 1% calf serum or 10% gamma globulin-free serum. The addition of hEGF to quiescent confluent monolayers of HF cells, maintained in a medium with 1% calf serum for 48 hours, resulted in a 10- to 20-fold increase in the amount of 3H-thymidine incorporation after 20-24 hours. The stimulation of thymidine incorporation was maximal at an hEGF concentration of 2 ng/ml, was dependent on the presence of serum, and was enhanced by the addition of ascorbic acid. In confluent cultures of HF cells, subject to density dependent inhibition of growth, hEGF was able to stimulate DNA synthesis more effectively than fresh calf serum. Human EGF stimulated DNA synthesis in quiescent cultures, however, regardless of cell density. The addition of rabbit anti-hEGF inhibited all effects of this growth factor on HF cells.