BACKGROUND To determinate the bactericidal activity of vancomycin, rifampicin, cefotaxime, ceftriaxone and their double and triple combinations against penicillin and third generation cephalosporin-resistant. Streptococcus pneumoniae strain. METHODS A study of the bactericidal activity of antibiotics alone and in combination was performed by killing-curves with fixed concentration of 2 micrograms/ml, equivalent to 1 < MIC for cefotaxime and ceftriaxone, to 8 x MIC for vancomycin and to 32 < MIC for rifampin. RESULTS Vancomycin was the only antibiotic with bactericidal effect at 6 hours. Cefotaxime had bactericidal effect at 12 hours. Rifampin had no bactericidal effect. The association of vancomycin plus third generation cephalosporins (cefotaxime and ceftriaxone) showed indifferent effect. Rifampin decreased the bactericidal activity of vancomycin at 12 hours, when both were studied in combination, and it demonstrated antagonistic effect in the double and triple combination in which rifampicin was included. CONCLUSIONS All associations of vancomycin plus third generation cephalosporins were bactericidal effects, but they did no show synergistic effects against penicillin- and third generation cephalosporin S. pneumoniae strain 2349. All combinations with rifampin showed decreased bactericidal activity or antagonistic effects. However, these results may be interpreted with caution because the in vivo experience of an experimental meningitis model in rabbits showed that the associations of vancomycin plus rifampin and vancomycin plus cefotaxime and rifampin were effective combinations. More in vivo and in vitro comparative studies are needed to demonstrate the reliability of in vitro results.