Measurement of minimal bactericidal concentrations of antibiotics is requisite to select the most appropriate drugs in severe infections. However, MBC assays are reliable only if the various technical modalities are well-standardized. We determined MBCs of ampicillin (E. coli), gentamicin (E. coli, P. aeruginosa, S. aureus), cefazolin (E. coli), ticarcillin (P. aeruginosa), and oxacillin (S. aureus), for different values of a number of parameters : inoculum from a 4 h and 18 h culture, measurement of MBC for 99.9% and 99.99% of bacteria, plate microdilution and macrodilution in plastic or glass test tubes, inoculation under a 1 ml or 0.1 ml volume, determination of MIC and subculture at the 24th hour, with or without Vortex agitation at the 20th hour. Most MBCs were identical for 99.9% and 99.99% of bacteria. Conversely, results were significantly modified by changes in other factors. Very high MBCs were consistently found in plastic tubes. MBCs were higher after inoculation under a 1 ml volume than under a 0.1 ml volume. MBC was usually similar to MIC with the microdilution technique, or with glass tubes, inoculation under a 0.1 ml volume and subculture at the 24th hour following Vortex agitation at the 20th hour.