The limited replicative life span of diploid human cells in vitro (cellular senescence) serves as a cellular model of aging. We examined the proliferative response of 2BS cells of different population doubling levels to epidermal growth factor (EGF). DNA synthesis was measured by thymidine incorporation. As the cells aged, there was a significant decrease both in the baseline level of DNA synthesis and in the stimulation of DNA synthesis by EGF addition. The effective concentration of EGF and the latent period prior to DNA synthesis did not change. EGF receptor mRNA expression also remained unchanged as the cells aged, in the absence or presence of EGF, suggesting that the defect in old cells lies downstream in the EGF signaling pathway. As the cells reached 100% of their life span, however, there was a 70% decrease in EGF receptor mRNA. Expression of the EGF receptor homologue HER-2 was also examined. The HER-2 mRNA level was significantly reduced in old cells. Moreover, HER-2 expression was stimulated by EGF addition in young cells but not in old cells. The results suggest that cell aging is associated with a progressive loss in the ability of cells to respond to growth factors.