Résumé : The diameters of the zones of inhibition of consecutive clinical isolates around antibiotic susceptibility test discs at medical centres in different parts of the world were computer filed and analysed by a series of programs that evaluate test quality and compare results. Percentages of isolates resistant to ampicillin at 18 centres ranged from 16 to 73 for Escherichia coli and from 3 to 56 for Proteus mirabilis. Percentages resistant to chloramphenicol ranged from 2 to 48 for E. coli, from 5 to 52 for Klebsiella pneumoniae, and from 8 to 67 for Serratia marcescens. Gentamicin resistance did not exceed 4% at any of 18 centres and was less than 1 at 14 of them for isolates of E. coli, while K. pneumoniae showed less than 2% resistance at six centres but averaged 22% at another eight. Multi-resistant isolates were ten-fold more frequent at eight centres than at the remaining six. Too few centres were sampled to characterize individual countries except in the United States where resistance seemed generally less prevalent. © 1986 The British Society for Antimicrobial Chemotherapy.