The roles of lipopolysaccharide and OmpC, a major outer membrane protein, in the receptor function for bacteriophage T4 were studied by using Escherichia coli K-12 strains having mutations in the ompC gene or in genes controlling different stages of lipopolysaccharide synthesis. The receptor activity for T4 was monitored by (i) T4 sensitivity of intact cells, (ii) phage inactivation activity of cell envelopes, and (iii) phage inactivation activity of specimens reconstituted from purified OmpC and lipopolysaccharide. It was found that (i) in the presence of the OmpC protein, the essential region of the lipopolysaccharide for the receptor activity was the core-lipid A region that includes the heptose region, whereas the glucose region was not necessarily required for the receptor function; (ii) the OmpC protein was not required at all when the distal end of the lipopolysaccharide was removed to expose a glucose residue at the distal end; and (iii) when cells lacked both the OmpC protein and the glucose region, they became extremely resistant to T4. Based on these findings, the roles of the OmpC protein and lipopolysaccharide in T4 infection are discussed.