GM2 containing NeuGc was a major ganglioside in the liver of mouse strains such as BALB/c, DBA/2, C3H/He, and C57BL/10, whereas WHT/Ht mouse liver did not contain GM2(NeuGc) but contained GM3(NeuGc) as a major ganglioside. Since GM3(NeuGc) is a biosynthetic precursor of GM2(NeuGc), WHT/Ht liver was considered to lack the ability to synthesize GM2(NeuGc) from GM3(NeuGc) (Hashimoto, Y., et al. (1983) J. Biochem. 93, 895-901). In this study we measured the activity of UDP-N-acetylgalactosamine : GM3(NeuGc) N-acetylgalactosaminyltransferase in the liver of BALB/c, WHT/Ht, and their progeny. The transferase activity in the microsomal fraction of BALB/c liver was 2.10 +/- 0.32 X 10(-5) units/mg protein (means +/- S.D.), whereas no activity was detected in that of WHT/Ht liver, F1 hybrids between BALB/c and WHT/Ht expressed GM2(NeuGc) as well as the enzyme activity, the level of which was almost half that in BALB/c liver 1.10 +/- 0.12 X 10(-5) units/mg protein). The backcross generation of F1 to WHT/Ht segregated into two groups with respect to expression of GM2(NeuGc) and the transferase activity: 11 of the 21 mice analyzed expressed both GM2(NeuGc) and the transferase activity (1.28 +/- 0.18 X 10(-5) units/mg protein), whereas the rest expressed neither. These results suggest that the expression of GM2(NeuGc) is directly regulated by the activity of UDP-N-acetylgalactosamine: GM3(NeuGc) N-acetylgalactosaminyltransferase in mouse liver.