The composition of structural proteins of lambdoid phages such as lambda, phi 80 434 divided by molecular weights was determined by means of SDS-disc-electrophoresis in a 15% polyacrylamide gel. The proteins of the same phages were divided by isoelectric points using an isoelectric focusing in a 5,25% polyacrylamide gel with 8 M urea and a gradient pH 7.0--3.5. The both methods brought out a composition character of the virion proteins and illustrated the high degree of similarity among the structural proteins of phages lambda and 434 and a far less similarity among the proteins lambda and phi 80. The antigenic composition of the lambdoid phage was determined and the basic antigenes were identified on one-dimensional and two-dimensional immunoelectrophoregrams. The appreciable immunochemical affinity of basic antigenes of the lambda and 434, but a partial affinity of the phages lambda and phi 80 were found. The basic protein of the head pE proved to be immunochemically similar for all three phages.