The severity of diabetes produced by the mutation diabetes (db) in the mouse is markedly strain-dependent. When the db mutation is maintained in the C57BL/KsJ (BKs) and DBA/2J strains, severe diabetes is observed, whereas when it is maintained in the C57BL/J (B6) inbred background, a mild, well-compensated diabetes is observed. Our studies on the regulation of malic enzyme activity showed that both the BKs and DBA/2J strains carried the b allele at the malic enzyme regulatory (Modlr) locus and had low enzyme activity, while the B6 strains carried the a allele and had malic enzyme activity two to three times that seen in the BKs and DBA/2J strains. To assess any role of malic enzyme activity in modulating diabetes severity, we produced an F2 generation of diabetic mice using BKs-db/+ and B6-db/+ mice as progenitors. Male diabetic mice of the F2 generation segregated into three groups with respect to diabetes severity. The concordance observed between diabetes severity and malic enzyme activity seen in these three groups suggests a major role for a gene (or a closely linked gene) regulating malic enzyme activity to be responsible for much of the genetic background effects observed with the db mutation in the mouse.