SummaryThe in vitro activity of aztreonam against 1836 Enterobacteriaceae and 350 non-fermenting Gram-negative bacilli has been examined by the Kirby-Bauer method in a multicenter study. The percentage of Enterobacteriaceae susceptible to aztreonam (96.5%) was similar to that of cefotaxime (93.7%) and ceftazidime (96.4%). The percentage of non-fermenters susceptible or partially susceptible to aztreonam (86.9%) was higher than that of cefotaxime (53.9%) but lower than that of ceftazidime (91.4%). Most strains resistant to either cefotaxime or ceftazidime showed cross-resistance to aztreonam.