SummaryNine enzyme activity variants and one charge variant of liver/erythrocyte pyruvate kinase have been found amongst laboratory and wild mice. Four of the enzyme activity variants were previously reported to be caused by allelic differences in the structural gene,Pk-ls. Analysis of two putative regulatory gene mutations is now reported, both of which map at, or close to, the structural gene on chromosome 3. One of these mutations, in the inbred strain SWR, is tissue specific, affecting enzyme concentration in the liver but not the erythrocyte the other, which arose in a mutation experiment, doubles the enzyme concentration in both tissues. The organization and the nomenclature in the [Pk-1] gene complex are discussed and are compared with the organization of other comprehensively analysed gene complexes in the mouse.