CLINICAL SPECIMENS sometimes contain strains of StaphyZococcus aureus that are composed of both antibiotic-resistant and antibiotic-sensitive cells; this has been interpreted as loss in vivo of genes coding for resistance, so that the sensitive cells are derivatives of the resistant (AylifEe and Collins, 1967; Annear and Grubb, 1969,1972; Noble, 1972). This interpretation is supported by the observation that the resistant organisms segregate sensitive derivatives in vitro. However, the results of experiments in vitro do not necessarily apply in vivo; and the mixed population of S. aureus in vivo might also have resulted from acquisition in vivo by a sensitive strain of genes coding for antibotic resistance. This has been observed to occur under experimental conditions (Novick and Morse, 1967; Lacey, 1971). In this paper we show that a strain of S. aureus initially resistant to five antibiotics lost these resistances in vivo. This loss of antibiotic resistance was probably a result of the exposure of the strain to cloxacillin. MATERIALS AND METHODS