Bacteriophage P1 has been shown previously to determine a product ban that can substitute in DNA replication for the protein specified by cis-tron dnaB of Escherichia coli. However, ban product furnished by P1 bac prophage (ban constitutive) substitutes only poorly for DNA replication in the absence of dnaB product in a strain bearing an unsuppressed amber mutation, dnaB266, as shown by the cryosensitivity of the dnaB266 (P1 bac) lysogen and its unability to support lambda growth. An additional mutation (termed crr) in the P1 bac prophage has been obtained which confers cryoresistance to the sup+ dnaB266 (P1 bac crr) lysogen and restores its ability to support lambda growth. ban product produced in P1 bac crr lysogen fulfills all dnaB roles in vivo, especially in the various instances in which ban product expressed in P1 bac lysogens does not. The ban product is expressed constitutively in P1 crr prophage. The crr-1 mutation is tightly linied to the bac-1 and ban-1 mutations and is dominant over crr+. The nature of the crr mutation is discussed: two hypotheses are considered, that of a mutation in the ban gene rendering the ban product more active or that of a site mutation in the ban operon increasing the level of ban expression. Expression of ban product (wild type or altered) leads to interactions with the variously altered dnaB product. Both positive and negative interactions are described. Genetic results presented here suggest that ban and dnaB subunits interact to form hybrid dnaB-like molecules; the average composition of which depends on the relative quantities of ban and dnaB subunits in the cell.