The replicative ability of variously ‘aged’ cell cultures, their insulin binding and biological responsiveness under control and glucocorticoid (i.e. hydrocortisone) amplified conditions have been studied in human fibroblast cultures. Insulin stimulation of DNA synthesis in early and late passage cultures and in cultures from young and old donors showed no age-related difference in insulin responsiveness. Hydrocortisone amplification of insulin-stimulated DNA synthesis in early and late passage cells expressed no age-related differences. Hydrocortisone affected basal DNA synthesis in cultures from in vivo young and old donors differently. Additionally, hydrocortisone amplified insulin-stimulated DNA synthesis in young donor cell cultures was observed to be higher than in old donor cell cultures. Specific 125I-insulin binding was increased by hydrocortisone in both early and late passage cultures and in cultures from young and old donors but no age-related differences in 125I-insulin binding were observed in the presence or absence of hydrocortisone. The data suggest that an age-related loss of an insulin postreceptor interaction during hydrocortisone amplification of insulin-stimulated DNA synthesis is being expressed in the cultures from old don