Fifteen strains of S. aureus resistant to fucidin were isolated from 17 patients during the routine examination of clinical specimens in 1971 (1·4 per cent.). Over half of them were associated with sepsis, but only 40 per cent. were isolated from patients who had previously been treated with fucidin. In-vitro studies indicate that the strains have a stable basic level of resistance with the potential to increase this resistance to high levels on exposure to fucidin. The strains multiply as rapidly as fully sensitive strains, may be associated with persistent and relapsing sepsis and may persist in the carriage state for up to 21 mth.