The type I restriction and modification genes of Escherichia coli can be transferred to other non-modified strains by conjugation without killing the recipient, implying that the restriction function must be regulated. In this study, two isogenic F' plasmids (r+K and r-K) served as donors in quantitative conjugation experiments with various restriction-deficient strains of E. coli and Salmonella typhimurium. Conjugation studies with hsd::lacZ operon fusions in F' plasmids indicate that both of the hsdK promoters, p(res) and pmod, express simultaneously following conjugative transfer. Thus these genes do not appear to be regulated at the transcriptional level. A spontaneous mutant of E. coli C was discovered that is presumably killed upon conjugative transfer of the hsdK genes (defined as a Crc- phenotype). The gene that is defective in the mutant was tentatively designated hsdC (control). Hfr gene replacement studies led to the localization of the putative hsdC gene between 6 and 16 min on the E. coli genetic map.