The susceptibility of an isolate of Rochalimaea quintana, Rochalimaea vinsonii and the newly recognized Rochalimaea henselae to antimicrobial agents was determined by dilution in Mueller-Hinton agar supplemented with 5% sheep blood. Two inocula of 10(2) and 10(4) cfu were used and tests were read after incubation at 5 and 7 days at 37 degrees C in an atmosphere enriched to 5% carbon dioxide. Each isolate was susceptible to amoxycillin, third-generation cephalosporins, tetracyclines, macrolides and rifampicin, with complete inhibition of bacterial growth at concentrations lower than 0.125 mg/L. Co-trimoxazole and aminoglycosides were also active. Conversely, MICs for oxacillin, cephalothin, clindamycin, chloramphenicol and the fluoroquinolones were around the maximum concentrations achievable in serum. Vanocomycin showed moderate activity with MICs of < or = 4 mg/L. These results are consistent with clinical experience in antibiotic treatment of R. henselae infections since failures were reported with first-generation cephalosporins and the isoxazolyl penicillins. However, the frequent relapses that are observed with other beta-lactams, tetracyclines or macrolides do not accord with the in-vitro susceptibility of Rochalimaea spp. Poor bactericidal activity as well as the intracellular growth of the bacteria within infected hosts, may explain such discrepancies.