Hydrocortisone (14 muM) added to cultures of human diploid fibroblast-like cells extends the lifespan of the population. This effect is expressed during a single growth cycle by an increased rate of proliferation and a higher rate of incorporation of 3HdT into DNA. The hormone appears to exert its effect by increasing the fraction of cells in the proliferating pool, and this increase is expressed at all levels of serial subcultivation (population ages). The effect requires a minimum of between 12 and 18 hr exposure of the cells to hydrocortisone. Studies with varying amounts of serum suggest that the hormone affects the cells by amplifying the serum signal which is the primary signal for cell division.