The aim of this study was to investigate the effect of interleukin-1 beta (IL-1 beta) on pulpal fibroblast proliferation and collagen synthesis in vitro. Cell proliferation was assessed by [3H]thymidine uptake at 24, 48, and 72 h. These cells were compared with dermal fibroblasts under a range of conditions. IL-1 beta inhibited pulp cell proliferation, but this effect was decreased by the presence of indomethacin. In contrast, IL-1 beta stimulated dermal fibroblast proliferation in the absence and presence of indomethacin. Collagen synthesis was analyzed by incorporation of 5-[3H]proline into polypeptide chains. IL-1 beta stimulated synthesis of type I collagen both in the absence and presence of indomethacin. Results suggest that the inhibitory effect on pulp cell proliferation is dependent upon IL-1 beta-induced prostaglandin E2 synthesis and that IL-1 beta is a potent mediator of prostaglandin E2 synthesis in dental pulp. These observations imply a role of IL-1 beta in collagen synthesis, but not in the fibroblast proliferation phase of the healing process.