Cell division by strains of Escherichia coli and Salmonella typhimurium is inhibited by 5-diazouracil (5-DU). Division recovers in the presence of the inhibitor after a period which is temperature-dependent. Recovery is probably due to breakdown of 5-DU and the rate of this breakdown is apparently increased at alkaline pH. Growth with 5-DU caused only a slight reduction in the rate of murein synthesis and no alteration in the properties or composition of membranes of S. typhimurium. The agent caused chaining in Streptococcus fecalis and inhibition of the penicillin-induced lysis of S. typhimurium. These effects may have been due to direct inhibition of lysin activity but an indirect effect seems more likely. The most marked effect of 5-DU on S. typhimurium was to cause a transient inhibition of DNA synthesis. Since 5-DU did not stop uncoupled cell division (i.e. division occurring independently of DNA replication) and since lon- strains were more sensitive to 5-DU than lon+ strains, it was concluded that 5-DU acts on cell division via an inhibitory effect on DNA replication.