The proteins synthesized in Escherichia coli B cells after infection with various T4 bacteriophage tail baseplate mutants were analysed by the immunoblotting method for the presence of the 15 Kilodalton lysozyme found in phage T4 particles. Using three different antisera: anti-phage, anti-baseplate and anti-15K lysozyme, it has been found that the 15K lysozyme is not present in lysates of bacteria infected with T4 gene 25 amber mutants. The 15K lysozyme was also found to be expressed in E. coli B cells transformed with a plasmid containing only a small portion of the T4 genome but which included T4 gene 25. These observations indicate that the 15K lysozyme is the gene 25 product.