Forty clinical isolates and the type strain of Nocardia brasiliensis were screened for susceptibility to 20 beta-lactams. Isolates exhibited a single pattern of resistance, with large zones of inhibition by disk diffusion and low MICs by broth and agar dilutions only to cefotaxime, ceftriaxone, ceftizoxime, Augmentin, and Timentin. All strains produced beta-lactamase, with five different enzyme patterns by isoelectric focusing. Despite the differences in their isoelectric points, the enzymes had the same substrate profiles, with equivalent activity against penicillin, ampicillin, cefamandole, cephalothin, and cephalordine. In an in vitro assay, the enzymes were highly susceptible to clavulanic acid. The MIC50 and MIC90 for the combination of amoxicillin and clavulanic acid (Augmentin) was 2 and 4 micrograms/ml, respectively, compared with 16 micrograms/ml for both values for amoxicillin alone. These studies suggest that beta-lactamase is the major mechanism of beta-lactam resistance in this species and that Augmentin is the first oral beta-lactam with good potential for treating infections due to N. brasiliensis.