Antistaphylococcal activity of three beta-lactam antibiotics, i.e. cefmenoxime, cefotaxime and latamoxef (moxalactam) was compared with that of nafcillin by studying cure rates of aortic valvular endocarditis caused by one of three clinical isolates of Staphylococcus aureus in New Zealand white male rabbits. The animals were randomly allocated to a control and four antibiotic treatment groups. Each animal except the controls received 60 mg/kg of one antibiotic intramuscularly twice daily for two weeks, beginning 24 h after induction of endocarditis. The aortic valve was sterilized in 11 of 15 (73%) animals treated with cefmenoxime, nine of 13 (69%) treated with cefotaxime, 11 of 13 (85%) treated with latamoxef and 12 of 15 (80%) treated with nafcillin. All nine animals in the control group developed aortic valvular vegetations with an average of 2.4 X 10(9) cfu/g. The differences in sterility rates resulting from treatment with the four antibiotics were not statistically significant (P greater than 0.70).