Seven beta-lactam antibiotics were studied for both their antimicrobial activity and the degree to which they produced inoculum effect on Escherichia coli, Klebsiella pneumoniae, and Salmonella typhimurium. Aztreonam, cefoperazone, and ceftazidime were poorly bactericidal, caused marked bacterial filamentation, and exhibited a large inoculum effect on E. coli, K. pneumoniae, and S. typhimurium. Cefotaxime and ceftriaxone were more rapidly bactericidal, caused only a moderate amount of filamentous forms, and exhibited a modest inoculum effect, while cefoxitin and imipenem both were rapidly bactericidal and exhibited only a minimal-to-no-inoculum effect. The inoculum effect did not correlate with drug stability during incubation with the bacteria. Inoculum effect on these species of the family Enterobacteriaceae appears to be a manifestation of increase in optical density secondary to the development of filamentous bacterial forms with an increase in bacterial mass during exposure to antibiotics which are not rapidly bactericidal. These observations have a clear significance for the susceptibility testing of beta-lactam antibiotics when turbidity is used as a parameter to determine presence of bacterial growth.