A growth factor is obtained from a human retinoblastoma cell line (Y-79) growing in culture in the absence of any added serum or hormone. The retinoblastoma derived growth factor (RDGF) is still secreted by the Y-79 cells after they have been growing in a serum-free environment for over 5 months. RDGF stimulates both cell division and thymidine uptake of Swiss mouse 3T3 fibroblasts in media containing 0.2% fetal bovine serum. The thymidine uptake occurs maximally 16-42 hr after addition of RDGF. The factor has an apparent molecular weight of 38,000 as determined by Sephadex G-100 chromatography and is heat and trypsin labile. There appears to be additional material of higher MW which also stimulates thymidine uptake by the 3T3 cells.