Ribonuclease III from Escherichia coli has been purified to apparent homogeneity by affinity chromatography on immobilized double-stranded RNA. Polyacrylamide gel electrophoresis of the purified enzyme in the presence of sodium dodecyl sulfate gave one band of protein with a molecular weight of approximately 25,000. Chromatography on Sephadex G-100 is consistent with a molecular weight of 50,000, suggesting that the native enzyme is a dimer. RNase III cuts some single-stranded RNAs, such as bacteriophage T7 early RNA, at specific sites in vivo. This RNA is cut as these same sites by the purified enzyme under all conditions tested. However, at low ionic strength relatively small increases in enzyme concentration produce cuts as secondary sites. At high ionic strength, the enzyme's preference for the sites cut in vivo is more pronounced and secondary cuts are made only at very high enzyme concentrations. Secondary cuts are shown to occur at specific sites and are made in a variety of RNAs even from sources other than E. coli. By cutting RNAs at secondary sites it should be possible to generate RNA fragments which would be useful in a number of studies.