Seventeen clinical isolates of Staphylococcus aureus (from the United States and Europe) selected for low (borderline)-level methicillin resistance (MIC of methicillin, 2 to 4 micrograms/ml; MIC of oxacillin, 0.5 to 8 micrograms/ml) were examined for their mechanisms of resistance. Five strains were typical of heterogeneous S. aureus: they gave positive reactions with a DNA probe specific for mec and contained a small fraction (10(-6] of highly resistant cells (MIC, greater than 100 micrograms/ml). The rest of the 12 strains were homogeneous with respect to their methicillin resistance: the MIC of methicillin for all cells was 2 to 4 micrograms/ml, and no cells for which MICs were 50 micrograms/ml or higher were detectable (less than 10(-9]. None of these strains reacted with the mec-specific DNA probe. One representative strain of each group was characterized in more detail. Strain CDC-1, prototype of heterogeneous methicillin-resistant S. aureus, contained penicillin-binding protein (PBP) 2a; its DNA could transform a methicillin-susceptible and novobiocin-resistant recipient to methicillin resistance with ca. 35% linkage to Novr. Introduction of the "factor X" determinant (K. Murakami and A. Tomasz, J. Bacteriol. 171:874-879, 1989) converted strain CDC-1 to high, homogeneous resistance. Strain CDC-6, prototype of the second group of isolates, showed completely homogeneous MICs of methicillin, oxacillin, and cefotaxime. The strain contained modified "normal" PBPs: PBPs 1 and 2 showed low drug reactivity (and/or cellular amounts), and PBP 4 was present in elevated amounts. No PBP 2a could be detected. DNA isolated from strain CDC-6 could transform the methicillin-susceptible and novobiocin-resistant strain to methicillin resistance in a multistep fashion, but this resistance showed no genetic linkage to the Nov marker. We suggest that staphylococci with borderline resistance may contain at least three different classes of mechanism: heterogeneous, methicillin-resistant S. aureus, PBPs of modified drug reactivities, and the previously reported hyperproduction of beta-lactamase (L.K. McDougal and C. Thornsberry, J. Clin Microbiol. 23:832-839, 1986).