In order to examine the in vivo activity of BMY-28100, serial quantitative nasal cultures were taken from 52 nasal carriers of Staphylococcus aureus following administration of the antimicrobial. Nasal carriage rates and mean log titers decreased significantly during (days 2-4 and 5-8) and following (days 1-2 and 6-7) treatment. No change in antibiotic sensitivities was noted in follow-up isolates, and the MIC90 remained at 2mcg/ml. BMY-28100 demonstrated good in vivo activity against Staphylococcus aureus.