The bfe locus codes for the cell surface receptor for vitamin B12, the E colicins, and bacteriophage BF23 in the Escherichia coli outer membrane. When the bfe+ allele, which is closely linked to the argH locus, was introduced into an argH bfe recipient by conjugation, arg+ recombinant cells rapidly and simultaneously acquired sensitivity to colicin E3 and phage BF23. In the reciprocal experiment introducing bfe into an argH bfe+ recipient, it was found that colicin E3-resistant, arg+ cells began to appear shortly after the arg+ recombinant population began to divide. This was far earlier than would have been predicted on the basis of 220 receptors per haploid cell. Moreover, there was a lag between the appearance of colicin resistance and the appearance of resistance to killing by phage BF23, and hence a period of time during which some arg+ recombinant cells were sensitive to the phage but resistant to the colicin. Colicin E3 added to cells during this period of time protected against phage killing, indicating that the colicin-resistant cells still had receptors capable of binding colicin on their surface. The modification of the phenotypic expression of colicin and phage resistance by inhibitors of deoxyribonucleic acid, ribonucleic acid, and protein synthesis was also investigated. The results obtained indicate that the receptor protein coded for by the bfe locus can exist on the cell surface in several different functional states.