A multicentre study of antibiotic susceptibility was performed in South Africa. Sensitivity to cephalothin, cefamandole, tobramycin and gentamicin was tested on a variety of aerobic and anaerobic bacteria. Two disc susceptibility techniques were used, i.e. the Kirby-Bauer technique (aerobes) and the broth-disc method (anaerobes); minimum inhibitory concentrations (MICs) were determined according to the International Collaborative Study techniques, and regression lines for individual centres were constructed. Satisfactory lines were obtained for cephalosporins, but, in some centres, problems were experienced with the aminoglycosides. Variations in MICs for Haemophilus influenzae were probably due to an inoculum effect. Accumulative percentage tables of the number of strains inhibited were compiled, and the comparative performance of the antibiotics was assessed.