Escherichia coli K-12 F- mutants defective in conjugation with an I-type donor (ConI-) were isolated and characterized. These mutants are specific in that they are conjugation proficient with other types of donor strains. They have an altered susceptibility to phages and detergents. Chemical analysis of the cell envelopes of mutant strains has shown that the lipopolysaccharide (LPS) is altered and that one major outer-membrane protein is absent. Conjugation experiments in which LPS from wild-type cells was added to a mating mixture, made up with wild-type donor and recipient cells, showed inhibition in transconjugant formation when an I-type donor, but not an F-type donor, was used. This strongly suggests that LPS of the recipient cell is directly involved in the ability to mate with an I-type donor but not with an F-type donor. The mutations are located in the 78- to 82-min region of the E. coli map, with one exception where the mutation maps near or in the galactose operon.