The Bgs locus determines tissue levels of beta-galactosidase in the mouse, so that enzyme levels are twice as high in mice carrying the Bgsh allele as in mice carrying the Bgsd allele (Felton et al., 1974). By immunotitration with antiserum to purified beta-galactosidase, we have found that the Bgs locus influences the amount of enzyme protein present in the tissues. We have utilized recombinant inbred lines derived from a cross between C57BL/6J and DBA/2J mice to confirm the location of the Bgs locus on chromosome 9. The inhibition of mouse beta-galactosidase by the active-site-directed reagent N-bromoacetyl-beta-D-galactosylamine has been investigated. beta-Galactosidase from the high and low Bgs strains has identical affinity for this inhibitor.