Whether a strain of methicillin-resistant Staphylococcus aureus susceptible to cephalothin by the disk-diffusion method was also susceptible to cephalothin in vivo was evaluated in the rabbit model of endocarditis. Rabbits with aortic-valve endocarditis due to methicillin-resistant S. aureus that were treated for four days with cephalothin had the same numbers of organisms in vegetations as did untreated rabbits. Treatment with cephalothin caused emergence of a highly resistant subpopulation in aortic-valve vegetations. Organisms highly resistant to cephalothin were also highly resistant to nafcillin. Thus broth-dilution and disk-diffusion tests may not predict therapeutic failure for cephalothin against strains of methicillin-resistant S. aureus. Because of cross-resistances among beta-lactam drugs, these strains should be considered uniformly resistant to this general class of antimicrobial agents, regardless of results from these tests.