The knockout mouse to alpha1,3galactosyltransferase (alpha1,3GT KO) lacks the ability to synthesize alpha-gal epitopes (Galalpha1,3Galbeta1,4GlcNAc-R) and is capable of producing low amounts of the natural anti-Gal antibody. The present study indicates that repeated immunization of these mice with rabbit red blood cell (RRBC) membranes results in production of anti-Gal in titers and specificity similar to those in humans. In contrast, immunized wild-type mice completely lack anti-Gal. Anti-Gal in the alpha1,3GT KO mice is produced in the circulation as the various IgG subclasses and as IgM isotype, but not IgA. In view of previous reports on the possible induction of T cell tolerance by immunization of mice with large amounts of antigen up to 24 days of age, we assayed possible induction of neonatal B cell tolerance toward the alpha-gal epitope. Eight-day-old neonates were subjected to immunization with 1 x 10(8) RRBC membranes, or 30 x 10(6) wild type mouse splenocytes, both of which express an abundance of alpha-gal epitopes. These neonatal exposures to alpha-gal epitopes did not prevent subsequent production of anti-Gal. Thus, the tolerance induction to this carbohydrate epitope is likely to be mediated by mechanisms other than those inducing neonatal T cell tolerance.