Comparison of eight lambdoid phages, including three Shiga-toxin converting phages, has been carried out with respect to the immunity region, especially the recognition helices of their repressor and CRO proteins on the one hand, and operator sequences on the other. Some as yet unassigned components of the regulatory circuits have been inferred by computer search. The cross immunity phenomenon shown by phages VT2-Sa and lambda is explained on the basis of similarity in their sequences. In addition, the similarity of 933W and HK022 in the sequences of their recognition helices of repressor and CRO, on the one hand, and operators, on the other, has led us to predict that they will have identical or similar immunity specificity. This homology has enabled us also to locate the OL (and consequently PL) of phage 933W that has been thought to be non-existent.
