When the concentration of hyaluronic acid was monitored in primary cultures of mouse skin dermal fibroblasts, there was an increase in hyaluronic acid proportional to the increase in cell number during the logarithmic growth phase. The concentration reached the maximum value 2 days before the cells became confluent, and then decreased gradually. Hyaluronic acid added at 1 mg ml-1 during the logarithmic phase either promoted or inhibited cell growth, depending on the density of cells at the time when hyaluronic acid was added. Hyaluronic acid (1 mg ml-1) added to subconfluent or postconfluent cultures induced a transient DNA synthesis with a consequent increase (greater than 20%) in cell number. The effects appeared to be specific, since neither hyaluronic acid oligosaccharides nor some other types of glycosaminoglycan (chondroitin, chondroitin sulphates, heparan sulphates and heparin) had any similar effects. Dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP), at 1 mM, added to subconfluent or postconfluent cultures had promoting effects successively on hyaluronic acid synthesis and on cell growth. An increase in hyaluronic acid synthesis also occurred when dbcAMP was added to day 1 cultures in the logarithmic growth phase, but the effect on cell growth was reversed; there was an inhibition rather than a promotion. The pattern of cell density-dependent variation of the dbcAMP effect is quite similar to that observed with exogenously added hyaluronic acid. Therefore, we propose that hyaluronic acid added exogenously or supplied endogenously by increased synthesis may act as a modulator of mouse dermal fibroblast proliferation.