In vitro performance of hydroxyapatite (HA)-reinforced polyethylene (PE) composite (HAPEX) has been characterized from both mechanical and biological aspects. The mechanical properties of HAPEX, such as tensile strength and Young's modulus, showed little change after immersion in a physiological solution at 37 and 70 degrees C for various periods. In addition, the biological response of primary human osteoblast-like (HOB) cells in vitro on HAPEX was assessed by measuring DNA synthesis and osteoblast phenotype expression. Cell proliferation rate on HAPEX was demonstrated by an increase in DNA content with time. A high tritiated thymidine ([3H]-TdR) incorporation/DNA rate was observed on day 1 for HAPEX, indicating a stimulatory effect on cell proliferation. The alkaline phosphatase (ALP) activity was expressed earlier on HAPEX than on unfilled PE and increased with time, indicating that HOB cells had commenced differentiation. Furthermore, it was found that the HA particles in the composite provided favourable sites for cell attachment. It appears that the presence of HA particles in HAPEX may have the advantage of acting as microanchors for bone bonding in vivo.