The preparation of active protein or proteins extracted from pituitary glands is called PF (pituitary factor). PF stimulates initiation of growth in different types of cells: 3T3 mouse fibroblast WI-38 human fibroblasts, Y-1 mouse adrenal cells and C-6 rat glial cells. Proteases like trypsin and thrombin showed no growth promoting activity under the conditions of PF assay. PMSF (phenilmethyl sulfonyl fluoride) treatment did not inactivate PF. No protease activity was detected in PF using BAPNA, azocasein and azoalbumin as substrates. It is then suggested that PF growth promoting activity is not due to proteases present in the preparation. Preliminary results are presented indicating that a functional rat pituitary cell line (GH3) might secrete a growth factor in culture.