A novel gene of Escherichia coli, rhtB, has been characterized. Amplification of this gene provides resistance to homoserine and homoserine lactone. Another E. coli gene, rhtC, provides resistance to threonine. The homologues of RhtB are widely distributed among various eubacteria and archaea, from one to 12 copies of family members that differ in their primary structure were found in the genomes. Most of them are genes that encode hypothetical transmembrane proteins. Experimental data that indicate participation of the rhtB product in the excretion of homoserine have been obtained.