Genetic variation of a new codominantly inherited esterase, designated ES-17, has been discovered in the house mouse using isoelectric focusing in polyacrylamide gels. The ES-17 A phenotype (three bands; isoelectric points, between pH 5.55 and pH 5.90) was found in C57BL/10Sn. LP/J possessed the Es-17B phenotype (three bands; isoelectric points, pH 5.05-5.55). ES-17 was present in all tissues examined, except for hemolysate and serum, and was most clearly expressed in the small intestine. Because of its reaction toward various substrates and inhibitors, ES-17 has tentatively been classified as acetyl esterase (EC 3.1.1.6). ES-17 was shown to be controlled by the structural locus Es-17, located on chromosome 9. From test-cross data, a gene order of Es-17-8.7 +/- 2.5 map units-Mpi-1-10.2 +/- 2.7 map units-Mod-1 was established.