The penicillin-binding proteins PBP 1A and 1Bs are the essential murein polymerases of Escherichia coli. Purification of these membrane-bound bifunctional transglycosylase-transpeptidases was a major obstacle in studying the details of both enzymatic reactions. Here we describe a simple, highly specific affinity chromatography method that takes advantage of the availability of the specific inhibitor of the transglycosylase site moenomycin A in order to enrich PBP 1A and 1Bs in one step from crude membrane preparations. Separation of PBP 1A from PBP 1Bs is achieved in a second step employing cation exchange chromatography yielding enzymatically active native murein polymerases.