The turbidities of cultures of bacterial were monitored continuously at 37 degrees C. in a biophotometer. In the early phase of logarithmic growth, at approximately 2 X 10(7) organisms per ml., antibiotic was added. A strain of Escherichia coli (E. coli) that was ampicillim-resistant, but cephaloridine-sensitive, produced beta-lactamase rapidly as shown by the breakdown of a chromogenic cephalosporin. The E. coli was lysed by cephaloridine 15 minutes after the addition of the antibiotic, but, even with a concentration 8-fold greater than the MIC, after 3.5 hours the antibiotic was not detectable in the culture medium and the strain had recommenced logarithmic growth. In the presence of a concentration of cephazolin 4-fold greater than the MIC the E. coli lysed after 30 minutes and did not recommence growth 6 hours, indicating much slower destruction of the antibiotic, presumably due to the greater beta-lactamase resistance of cephazolin. Similar results were obtained with other E. coli and a strain of Klebsiella pneumoniae.