AbstractSeveral cell types such as Balb/c 3T3 have been shown to require platelet‐derived growth factor (PDGF); however, strains of human fibroblasts from fetal donors have been shown to divide in medium containing plasma free of PDGF. Since human fibroblasts have been demonstrated to secrete other peptide growth factors such as somatomedin‐C, we have undertaken a study to determine if fibroblasts derived from fetal donors are capable of producing a mitogen(s) which will substitute for PDGF and support growth in plasma alone. Quiescent human fibroblasts from donors ages 12‐wk embryo, newborn, and 3‐yr‐old were exposed to serum‐free minimum essential medium (MEM) for 24 hr. The conditioned media collected from embryonic and newborn fibroblast donors were demonstrated to stimulate growth in the 3‐yr‐old cells with the addition of plasma alone, whereas conditioned medium from the 3‐yr‐old donor cells was without effect. The increases in growth and DNA synthesis were dependent upon concentration of media used. Conditioned medium derived from newborn fibroblasts also supported 3‐yr‐old cell growth but embryonic conditioned medium was more potent. The embryonic conditioned medium factor was heat and acid stable but destroyed by trypsin and excluded by a 5,000 (MW) molecular weight filter. The factor(s) had full competence factor activity since transient exposure to fibroblasts (3‐yr‐old donor) stimulated 78% nuclear labeling vs. 81% with continuous exposure. These results support the concept that there is an age‐dependent production of a competence factor by human fibroblasts which may partially account for their capacity to grow in medium devoid of PDGF and s