The determination of serum concentrations of antimicrobials with a narrow therapeutic range by microbiological method requires some modifications when the patient is under combined antimicrobial therapy. The methodological conditions for the appraisal of aminoglycosides and vancomycin in the presence of other drugs were evaluated by using strains with selective sensitivity to the antimicrobial to be tested or by the adding crude extracts of beta-lactamases for the inactivation of the beta-lactam antimicrobials. These beta-lactamases were obtained from Enterobacter cloacae P99 (derepressed mutant) cultures for the inactivation of penicillins and first, second and third generation cephalosporins, as well as from Stenotrophomonas (Xanthomonas) maltophilia M 1484 cultures for the hydrolysis of imipenem. The strains allowing vancomycin appraisal and indifferent to the synergic activity of other drugs in the mixture were S. aureus M 2120, when the accompanying antimicrobials were gentamicin, ciprofloxacin or rifampicin, and E. faecalis M 2113, E. avium M 2091 and S. aureus M 2101 when the drugs in the mixture were amikacin, lincomycin or trimethoprim-sulfamethoxazole, respectively. For the appraisal of gentamicin in the presence of vancomycin or fluoroquinolones (ciprofloxacin or norfloxacin), E. coli M 1376 was the most suitable strain, while the use of K. pneumoniae ATCC 10031 was required for the appraisal of amikacin or netilmicin. When rifampicin was the accompanying antimicrobial, E. coli M 1495 turned out more adequate. E. coli M 1462 proved to be more satisfactory for gentamicin, amikacin and netilmicin dosages in samples also containing chloramphenicol, trimethoprim-sulfamethoxazole or tetracycline.