Systemic tolerance to BLG was studied in a) male and female mice heterozygous for the BLG transgene derived from crossing homozygous BLG transgenic males with wild-type females, b) BLG-transgenic and non-transgenic offspring derived from mating male and female heterozygous for the BLG transgene with wild type partners and c) BLG-transgenic and non-transgenic mice derived from back crossing onto a CBA/Ca MHC background. Using either a BLG-specific ELISA (for antibody responses) or footpad thickening assay (for T cell responses) the immune response to the BLG antigen was assessed. Hypo-responsiveness to both ovine and bovine BLG was observed at the antibody level but not the T cell level for mice transgenic for BLG as compared to wild-type and non-transgenic littermates. Antibody tolerance could not be attributed to expression of the gene during pregnancy and lactation since all mice tested were virgin mice. These experiments also confirm that suckling on BLG-containing milk was not responsible for the antibody hypo-responsiveness seen in BLG-transgenic mice. Male and female mice heterozygous for the BLG transgene were mated to wild type partners such that the offspring fell into eight classes: male or female, suckled or non-suckled on 'transgenic' milk and heterozygous or wild-type for the transgene. Antibody and T cell data indicated that suckling 'transgenic' milk did not induce oral tolerance to ovine BLG in either transgenic or non-transgenic offspring. In contrast, voluntary ingestion of bovine BLG by wild-type mice for 24 hours or 21 days resulted in both antibody and T cell tolerance. Hypo-responsiveness could not be induced by transferring transgenic marrow into lethal irradiated normal recipients, but similarly irradiated transgenic recipients were still hypo-responsive after reconstitution with normal bone marrow.