Acetoxyoxamide (AOA), a derivative of hydroxyurea, was shown to inhibit the growth of a number of gram-negative bacteria; inhibition of growth was accompanied by extreme elongation of individual cells, characteristic of unbalanced growth. Analyses of nucleic acids and protein in Escherichia coli grown in the presence of low concentrations of AOA revealed a moderate reduction of the DNA/protein ratio, a more marked reduction of the DNA/RNA ratio, and an increase of the RNA/protein ratio. DNA synthesis in Ehrlich ascites tumor cells was inhibited by AOA, and the onset of inhibition was time-dependent and first order in character. Only nominal effects were found on RNA and protein synthesis. An enzyme in liver slices and in ascites tumor cells apparently hydrolyzed AOA to oxamyl hydroxamic acid (OHA). No such enzyme was found in erythrocytes or in serum. It was concluded that the selective action of AOA on DNA synthesis depended on its hydrolysis to OHA.