In Lactobacillus casei S-I, D-galactosamine and L-rhamnose comprised a phage receptor for phage J-I. A mixture of D-galactosamine and L-rhamnose effectively inactivated phage J-I, and a J-I-resistant mutant strain, L. casei S-I/J-I, lacked D-galactosamine in its surface component. The phage-inactivating effects of D-galactosamine and L-rhamnose were strongly dependent on the concentration of each substance and on temperature. It is suggested that the receptor for phage J-I involves both D-galactosamine in the cytoplasmic membrane and L-rhamnose in the wall of the host bacterium L. casei S-I, which lacts teichoic acid in its wall.