Microparticle induced inflammatory reaction has been extensively studied as a potential cause of implant loosening. However, there has been little in vivo study on the effect of the particles on the preserved cartilage in partial joint replacements. The purpose of this study was to determine in vivo effects of microparticles on the articular cartilage. Ninety rabbit knee joints were challenged repeatedly with 1-3 micron commercial pure titanium (Ti) and 1-45 micron ultra high molecular weight polyethylene (PE). After 12 weeks, patella and distal femur were harvested for determination of degenerative change by light and scanning electron microscope. The results are as follows: 1, In the joints without exposure of the subchondral bone, neither Ti nor PE elicited significant change. However, simultaneous introduction of both particles resulted in significant degeneration of the articular cartilage. 2. With the subchondral bones exposed, Ti and PE both induced significant cartilage degeneration. In this condition the PE particles were more detrimental than the Ti particles in causing degeneration of the articular cartilage. Although there exists a species difference, these results imply that the longevity of partial joint replacements may be shortened not only by mechanical problems, but also by the microparticles causing secondary degenerative change.