We evaluated the in vitro antibiotic susceptibilities of 31 coagulase-negative Staphylococcus isolates causing septicemia in neutropenic patients undergoing norfloxacin prophylaxis. All the strains but one were resistant to 1 microgram of norfloxacin per ml. At the same concentration, ciprofloxacin, ofloxacin, imipenem, and pefloxacin were inhibitory for 19 (61%), 19 (61%), 18 (58%), and 14 (45%) of the evaluated strains, respectively. Imipenem had an MBC/MIC ratio of greater than or equal to 32 against 19 (61%) of the evaluated isolates, and resistant subpopulations were detected at 5 micrograms/ml in 16 of 17 oxacillin-resistant strains and in 3 of 14 oxacillin-susceptible or -tolerant strains. Resistance to gentamicin was seen with increased frequency among slime-producing strains.