AbstractWe have examined the effects of in vitro aging on the growth capacity of human umbilical vein endothelial cells (HUVECs) under the influence of tumor necrosis factor (TNF) with or without interferon‐γ (IFN‐γ). The growth and colony‐forming abilities of control cells were impaired with advancing age in vitro, especially at later stages (more than 70–80% of life span completed). It was found that treatment with TNF inhibited growth and colony‐forming efficiency at any in vitro age. The effects of TNF were shown to increase with increasing in vitro age, as reflected by a more pronounced increase in doubling times, a decrease in saturation density, and a reduction in colony‐forming efficiency. However, the characteristics of TNF receptors, including the dissociation constant, and the number of TNF‐binding sites per cell‐surface area remained rather constant. The effect of TNF was augmented by IFN‐γ at a dose that alone affected growth and colony formation only slightly. The augmentation by IFN‐γ was also found to depend on in vitro age; the synergy with TNF in the deterioration of colony‐forming ability was observed only in “aged” cells. These results suggest that the intrinsic responsiveness of HUVECs to growth‐inhibiting factors, as well as to growth‐stimulating facto