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There has been a revived interest in metal–metal total hip replacements because of their potential for improved wear performance compared with conventional metal–polyethylene implants. The aim of the present study was to characterize metal wear particles isolated from metal–metal hip simulator testing of various clinically relevant alloys and to analyze the effects of these alloys and the number of loading cycles on wear particle characteristics. Implants were manufactured using medical-grade cobalt–chromium–molybdenum (CoCrMo) alloys that were high-carbon wrought, low-carbon wrought, or cast (with solution annealing). Testing was performed in a MATCO orbital bearing hip simulator in 95% bovine calf serum. The wear particles were isolated from the serum at test periods of 0–0.25 million cycles (Mc) (run-in wear) and 1.75–2 Mc (steady-state wear) using an enzymatic protocol previously optimized to minimize particle changes due to reagents. Isolated particles embedded in epoxy resin were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDXA). The EDXA results revealed the predominance of “lighter” particles containing Cr and O (most likely chromium oxide particles from the passivation layer) and fewer darker CoCrMo particles, with varying ratios of Co and Cr (possibly from carbides and from implant matrix material). More CoCrMo particles were observed with the low-carbon wrought alloy, but the majority of the particles for all three alloys was chromium oxides, especially for the 1.75–2 Mc test period. Image analysis of TEM micrographs revealed that for 0–0.25 Mc, there was up to 21% needle-shaped particles but that the majority remained round to oval in shape, reflecting the predominance of chromium oxide particles. Particle length averaged about 52 ± 4 nm, with only small differences due to the alloy. For 1.75–2 Mc, most particles were round to oval in shape. They were even less needle-shaped than at 0.25 Mc, and they had a slightly smaller length, averaging 46 ± 3 nm. In addition to characterizing the size and shape of particles from a MATCO simulator, this study is the first to demonstrate that particles that do not contain Co (presumably chromium oxides) can be predominant in the wear of metal–metal hip implants. It is therefore recommended that future in vitro and in vivo studies include the effects of these particles rather than just the effects of CoCrMo particles on the overall tissue response. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 67A: 312–327, 2003 [ABSTRACT FROM AUTHOR]