The report of Carrel and Ebeling (J. Exp. Med., 34 (1921) 599-623) generally gives the impression that both serum and blood plasma from old animals inhibit cell proliferation. For confirmation of this, we examined the effects of serum from rabbits of various ages on rabbit fetal skin fibroblasts (RSF cell) and human fetal lung fibroblasts (TIG-1 cell). Serum from young rabbits 8 months of age stimulated proliferation of RSF cells just as did fetal bovine serum, but that from old rabbits 5-7.8 years of age was found to significantly increase proliferation more than serum from the young. This was also the case when using TIG-1 cells. The lesser effect on cell proliferation by young serum apparently does not arise from growth-inhibitory factor(s) in the blood components. An examination showed young serum to possibly contain fewer growth-stimulatory factor(s) than old serum. On the basis of our data, we concluded that old rabbit serum stimulates, not inhibits, the proliferation of RSF and TIG-1 cells.
Aging Animals