Continuous (long-term) exposure of cultured normal (diploid) human fibroblasts (WI-38) to hydrocortisone (1.4 X 10(-7) M) resulted, as originally described by Macieira-Coelho (1966) and Cristofalo (1970), in a stimulation of proliferative activity and an increase of population doublings. Stimulation of DNA-synthesis by hydrocortisone, as measured by 3H-thymidine incorporation, required the presence of serum in the culture medium. Analysis of the cellular glycosaminoglycan (GAG) pattern, as measured by 14C-glucosamine incorporation into the various GAG types (hyaluronic acid, heparan sulfate, chondroitin sulfate, dermatan sulfate) revealed a significant increase of cell-bound hyaluronic acid (it appears to be predominantly located at the cell surface or pericellular, since it is removable to a large extent by trypsin treatment), while the distribution pattern of sulfated GAGs did not exhibit a significant change. This increase of cellular hyaluronic acid synthesis was regarded largely as an adaptive response to hydrocortisone, since its removal from the culture medium of hydrocortisone pretreated cultures resulted in a significant decrease of cellular hyaluronic acid. Possible functions of cell-bound hyaluronic acid were suggested in regard to cell surface properties (cell-cell and cell-substratum adhesion; migratory activity). Thus, decreased adhesiveness (by elevated cellular hyaluronic acid) might be a decisive factor for the well-known increase in cell saturation density caused by hydrocortisone. Generally, the present findings support a concept (Sluke et al., 1981; Schachtschabel and Sluke, 1984) that an increase of cellular hyaluronic acid synthesis is "growth-favorable", which is in line with previous findings of a decrease of hyaluronic acid synthesis by growth restriction in the course of in vitro ageing (Sluke et al., 1981).