This communication describes a method for the stimultaneous purification of Escherichia coli termination factor rho, RNAase III and RNAase H, which is rapid, reproducible and high in yield. Depending on how cells are grown 0.5 to 1 mg of RNAase III, 1 to 2 mg of RNAase H and 1 to 2 mg of rho are obtained from 100 g wet cells. RNAase III and rho are pure proteins, and RNAase H 80% pure. In addition it is shown that pure RNAase III degrades only RNA. RNA duplexes, and is responsible for the sizing of early T7 mRNA. The active form of RNAase III is composed of two identical subunits having a molecular weight of 23 500. Native RNAase H which specifically hydrolyses the RNA moiety of an RNA. DNA hybrid, is a single polypeptide chain with a molecular weight of 21 000 The amino acid composition of termination factor rho is also reported.