The growth of DBA/2 mouse embryo fibroblasts, as well as their prostaglandin (PG) production, was compared under 3 different culture conditions: RPMI 1640 supplemented with 10% fetal bovine serum (FBS), 2% Ultroser SF (steroid-free) or with 2% Ultroser G (containing steroids). The effect of the absence or presence of glucocorticoids on both parameters was more precisely investigated. In FBS-supplemented cultures, dexamethasone had a stimulatory effect on cells characterized by a slow growth rate, whereas it markedly inhibited proliferation in rapidly growing fibroblasts. The experiments carried out with serum substitutes (Ultroser SF and G) strongly corroborated the role of the absence or presence of glucocorticoids on fibroblast proliferation. Manipulations of glucocorticoid concentrations in Ultroser SF by adding 5 x 10(-8) M dexamethasone or in Ultroser G by adding 10(-6) M RU 486 reversed the effect of the absence of glucocorticoid in the first case, or in the latter case the effect of the presence of glucocorticoid on both cell growth and PG production. Progesterone had no effect by itself. Our results emphasize the importance of performing complete kinetic studies to investigate the effect of a given factor on cell proliferation in vitro, since glucocorticoids may have opposite effects on fibroblast proliferation according to their cell growth pattern in vitro.