1. Two components of colicin E3, namely proteins A and B, were prepared by means of an improved method.
2. Protein A thus obtained was more than a thousand times as active as native colicin E3 when they were assayed in terms of activity for ribosome inactivation.
3. Protein A was reconstituted to colicin E3 simply by mixing with protein B.
4. Trypsin digestion of colicin E3 yielded two fragments, T1 and T2, probably by cleaving one specific bond of the A moiety of colicin E3.
5. T2 was a complex of T2A and B proteins. T2A showed an activity equivalent to that of protein A when assayed in the in vitro system, and its activity was neutralized by protein B. Thus T2A was assigned as an active fragment of protein A.
6. T2A has a characteristic amino acid composition rich in the basic amino acid, Iysine.
7. The structure and function of the colicin E3 molecule is discussed based on the results obtained with its components as well as with fragments of the components.