A thermosensitive mutant of Escherichia coli K-12 was isolated in which the membrane fractions were deficient both in penicillin-binding protein-1Bs, the major components of protein 1 [Spratt, B.G. & Pardee, A.B. (1975) Nature 254, 516-517] and in activity for in vitro peptidoglycan synthesis. The mutant was also supersensitive to many kinds of beta-lactam antibiotics. All these phenotypic changes were found to be caused by a single mutation (mrc). Genetic mapping studies show that the mrc mutation was located at about 3.3 min on the E. coli chromosome linkage map [Bachmann, B.J., Low, K.B. & Taylor, A.L. (1976) Bacteriol. Rev. 40, 116-167]. Penicillin-binding protein-1Bs seemed to be identical to one of the essential enzymes involved in crosslinking of peptidoglycan and the target of cell-lytic action of penicillins. Possible functions of some other penicillin-binding proteins in compensating for lack of protein-1Bs were also proposed.