The in vitro activities of various quinolones, two newer cephalosporins, and imipenem against 23 strains of Nocardia asteroides, 4 strains of Nocardia brasiliensis, and 4 strains of Nocardia caviae were determined by an agar dilution method. PD-117558, a newer carboxyquinolone, and imipenem were the most active agents tested, inhibiting 90% of N. asteroides isolates at an MIC of 2 micrograms/ml. Of the 23 strains of N. asteroides, 15 were susceptible to cefpirome and 10 were susceptible to ciprofloxacin. The N. brasiliensis and N. caviae strains were very susceptible to PD-117558 (MIC, less than or equal to 1 microgram/ml), moderately susceptible to ciprofloxacin, and resistant to most of the other tested drugs.