Studies were attempted to know biochemically on bacteriophage T(4) head proteins, especially those which have been known to participate in its head morphogenesis. Both IPI and IPII were proved to be cleaved and the cleavage of all the following proteins, P23, IPI, IPII and IPIII, was proved to occur near to their N-terminals during this morphogenesis. In vitro examination revealed that this cleavage is caused by some enzymatic reaction. Amino acid compositions of P23(c), P24, Wac and the phage-λE protein were rather similar to each others as the structural proteins, while P22, IPI(c), IPII(c) and IPIII(c) were quite rich in lysine and were basic as the nuclear protein histon. The preliminary examination for the sequencing protein structure was attempted on the major protein, P23 and P23(c).