Two groups of 38 patients have been treated for staphylococcal infection with either cephaloridine (4 gm daily) or gentamicin (320 mg daily) by the intramuscular route. The rate of favorable clinical response was higher among the patients who received cephaloridine (78.8 per cent) than among those who were treated with gentamicin (60.5 per cent). No death related to the infection occurred in the cephaloridine-treated patients. The mean peak and trough antibacterial activity reached in the serum of the patients after injection of the antibiotics was higher in patients receiving cephaloridine (1/64 and 1/16) than in those treated with gentamicin (1/16 and 1/4). Patients who failed to respond to therapy had often a low antibacterial activity of the serum. These studies suggest that the 1/8 level of bactericidal activity should be attained in the serum one hour after the administration of the antibiotics to allow optimal results in staphylococcal infections.