The observed suppression of the temperature sensitivity of chromosomal DNA synthesis in various dnaB mutants of Escherichia coli by a variety of large conjugative antibiotic. Resistance conferring plasmids (R factors) had been postulated to be due to the presence on these plasmids of a dnaB analog ( ban) gene. Attempts to support this hypothesis by physically locating and characterizing this postulated gene of the R factor R64-ll by means of~ vitro recombinant DNA and ON/\ hybridization techniques were unsuccessful. It was observed that no regions of this R factor DNA cloned into a multi-copy plasmid vector exhibited suppressing activity when genetically transformed into several dnaB temperature sensitive mutants , nor was there any detectable nucleotide sequence homology between the dnaB gene of E. coli and -- --R64-ll. Further investigation of this R factor- mediated suppresion phenomena provided data which tended not only to dispute the ban gene hypothesis; at least with respect to R64-ll, but to suggest that this suppression might be due to a generalized 'mutator' property of this plasmid.