Cefoxitin, cefuroxime, and cephalothin were added to dense populations of beta-lactamase-producing enterobacteria, and the subsequent turbidity changes were monitored continuously. Viable counts and antibiotic assays were made at intervals after the addition of antibiotic, and the morphological appearances of the organisms were observed. Cephalothin caused lysis of most of the organisms tested, but even at high concentrations, after a few hours the antibiotic was destroyed and the organisms recommenced logarithmic growth. Cefoxitin produced lysis of all the strains of Escherichia coli and Klebsiella species tested, with supression of regrowth. With cephalothin and cefoxitin the viable counts after the addition of antibiotic correlated with the turbidity measurements. Cefuroxime infrequently caused lysis that suppressed multiplication, and the organisms became long and filamentous while the turbidity readings increased; the viable counts did not correlate with the turbidity measurements. Cefuroxime and cefoxitin were not destroyed by the beta-lactamases of any of the strains of enterobacteria that were studied.