Bovine synovial fibroblasts in primary monolayer culture were exposed to particulate metallic debris. The effects of the metallic particles on the synthesis and secretion of proteolytic enzymes and on cell proliferation and viability were examined. Uniform suspensions of titanium, titanium-aluminum, cobalt, and chromium particles, ranging in size from approximately 0.1 to ten micrometers (average, one to three micrometers), were prepared; the particle concentrations (the volume of particles divided by the total volume of the suspension) ranged from 0.0005 to 5 per cent. Aliquots of the particle suspensions were added to the synovial fibroblast cultures. The final particle concentrations in the media ranged from 0.0000083 to 0.83 per cent. After seventy-two hours of exposure, each medium was harvested and was assayed for proteolytic and collagenolytic activity and for hexosaminidase levels. Neutral metalloproteases, quantified by collagenolytic and caseinolytic (proteolytic) activity, represent enzymes, secreted by cells, that are capable of degrading extracellular matrix. Hexosaminidase is a marker for lysosomal enzyme activity that can include more than thirty enzymes, such as proteases, lipases, nucleases, and phosphatases. Cell proliferation was quantified by uptake of 3H-thymidine. Cell morphology was examined by scanning electron microscopy. Titanium, titanium-aluminum, and chromium significantly stimulated 3H-thymidine uptake at low particle concentrations (p < 0.01, p < 0.002, and p < 0.002, respectively). Exposure to cobalt, even at the lowest particle concentration, resulted in a significant decrease in thymidine uptake (p = 0.027). At the highest particle concentrations, all particles were toxic, as evidenced by the absence of thymidine uptake. At high particle concentrations, all of the metals caused a decrease in caseinolytic (proteolytic) and collagenolytic activity in the culture media. Titanium elevated the lysosomal enzyme marker, hexosaminidase, except at high concentrations. Chromium and titanium-aluminum had no significant effect on hexosaminidase at any particle concentration, while cobalt decreased all enzyme markers at mid-particle to high-particle concentrations. Scanning electron microscopy demonstrated that the morphological response of fibroblasts to titanium included membrane-ruffling and extension of filopodia, typical of active fibroblasts. In contrast, exposure to cobalt at the same concentration resulted in cell crenation, indicative of cell death.