When, in vivo, calcium hydroxide [Ca(OH)2] or hydroxyapatite are used as dental pulp-capping agents, a reparative dentine bridge is observed. New hard tissue is formed directly on the hydroxyapatite, whereas a characteristic necrotic area appears under Ca(OH)2. The differing pulpal reactions to these two capping agents suggest differing cell responses. After isolation and selection of human pulp fibroblasts in vitro, the cells were characterized by their morphology, their high alkaline phosphatase specific activity, and their synthesis of type I and III collagens and fibronectin. They were then incubated in the presence of either hydroxyapatite (1 mg/ml) or Ca(OH)2 (0.8 mg/ml). With Ca(OH)2, the cells exhibited dramatical alterations in morphology, DNA synthesis, alkaline phosphatase activity and protein synthesis, in accordance with the necrosis observed in vivo. With hydroxyapatite, phagocytic activity of pulpal fibroblasts toward hydroxyapatite particles (< 10 microns) was seen. As a consequence, DNA synthesis was affected. This inhibitory effect was not due to cell damage, as demonstrated by increased [3H]-proline and [3H]-leucine incorporation by the cells. There was also an inhibitory effect of hydroxyapatite on alkaline phosphatase activity, suggesting that the pulp fibroblasts were not in a differentiation stage. In conclusion, compared to the effects of Ca(OH)2 on human pulp fibroblasts, these data are consistent with the biocompatibility of hydroxyapatite previously described in vivo and testify to the occurrence of a biological response elicited by this synthetic biomaterial.