The radioactively labeled proteins synthesised in Escherichia coli minicells infected by bacteriophage lambda R and lambda R+ were compared by polyacrylamide gel electrophoresis. lambda R mutants, which have lost the ability to lyse host cells, lack a polypeptide of molecular weight 17.5 KD corresponding to the molecular weight of murein transglycosylase - a bacteriolytic enzyme from lambda lysates which we have described previously. It has been shown by direct comparison using radio-labeled enzyme that transglycosylase comigrates with the R gene product. The enzyme was undetectable in induced cultures of E. coli W3350 su degrees (lambda cI857 Ram 5) and C600 (lambda cI857 acR301), while it was present in a lambda Rz mutant lysate. We conclude that the transglycosylase is the R gene product.