Hayflick and Moorhead demonstrated that diploid human fibroblasts have a limited life span when grown in media containing 10% bovine calf sera. Recent experiments have suggested that antigrowth factors in serum may be a potential contributor to the limited proliferative capacity of normal diploid cells. To reduce the concentration of inhibitory serum factors 10-fold, MRC-5 diploid fibroblasts were cultured in media with only 1% serum. Long-term culture in 1% serum requires the addition of purified growth factors to sustain proliferation. Although there are dramatic changes in cell morphology, we find that the long-term division potential of MRC-5 cells cultured in media containing 1% serum and growth factors differs little from that found with cells cultured in 10% serum. In contrast, MRC-5 cells cultured in 10% serum and added growth factors have a somewhat extended life span. These results suggest that negative growth factors are not responsible for the limited proliferative capacity of in vitro cultured human fibroblasts. Moreover, the evidence that human fibroblasts can undergo major changes in cell morphology and still retain a normal life span raises questions about the validity of using morphological changes as indicators of cellular senescence.