The antigenic composition of the bacteriophages lambdaC1857 and lambdagt-lambdaC was investigated by modified immunoelectrophoresis in a 1,2% agarose gel involving 1% Triton X-100 and 0,25% sodium desoxycholate. The phages lambdaC1857 and lambdagt-lambdaC were shown to have identical antigenic compositions and to comprise three basic antigens, such as a1, a2, a3. The main structural proteins of the phage such as pE, pV and pD were isolated by preparative electrophoresis in 13% polyacrilamide gel. The immunophoresis of individual proteins indicated the antigens a1, a2 and a3 to be proteins pV, pD and pE, respectively.