AbstractThe effect of epidermal growth factor (EGF) on the in vitro growth of human malignant tumors was compared in serum‐supplemented (n = 54) and serum‐free (N = 41) media at clonal density to determine the true EGF dependency of tumors. In the complete absence of serum at a 1,000 cells/cm2seeding inoculation (approximately 100–200 adherent cells), EGF increased growth by>50% in 27 of 41 specimens (66%), and growth increased by 100% or more in 18 of these EGF‐sensitive tumors. In 12 serum‐free cultures (29%), in vitro growth failed to occur without EGF. With 10% serum supplementation and a lower cell density (250 cells/cm2), EGF increased growth by>50% in 34 of 54 specimens (63%), of which 25 had more than a 100% increase. The maximum growth induced by EGF in serum was usually seen in those tumors already capable of moderate in vitro growth. No difference in response to EGF was detected between specimens from primary tumors (n = 24) and those from metastases (n = 30).Under the stringent culture conditions of complete absence of serum and with tumors seeded at a low cell number, EGF stimulated most primary or metastatic human tumors to establish and sustain short‐term in vitro growth s