Background. The expression of galactoseα1,3galactose (Gal) in pigs has proved a barrier to xenotransplantation. Miniature swine lacking Gal (Gal-/- pigs) have been produced by nuclear transfer/embryo transfer. Methods. The tissues of five Gal-/- pigs of SLA dd haplotype (SLAdd) were tested for the presence of Gal epitopes by staining with the Griffonia simplicifolia IB4 lectin. Their sera were tested by flow cytometry for binding of IgM and IgG to peripheral blood mononuclear cells (PBMC) from wild-type (Ga+/+) SLA-matched pigs; serum cytotoxicity was also assessed. The cellular responses of PBMC from Gal-/- swine toward Gal+/+ SLA-matched PBMC were tested by mixed leukocyte reaction and cell-mediated lympholysis assays. Results. None of the tissues tested showed Gal expression. Sera from all five Gal-/- pigs manifested IgM binding to Gal+/+ pig PBMC, and sera from three showed IgG binding. In all five cases, cytotoxicity to Gal+/+ cells could be demonstrated, which was lost after treatment of the sera with dithiothreitol, indicating IgM antibody-mediated cytotoxicity. PBMC from Gal-/- swine had no proliferative or cytolytic T-cell response toward Gal+/+ SLA-matched PBMC. Conclusions. Gal-/- pigs do not express Gal epitopes and develop anti-Gal antibodies that are cytotoxic to Gal+/+ pig cells. The absence of an in vitro cellular immune response between Gal -/- and Gal+/+ pigs is related to their identical SLA haplotype and indicates the absence of immunogenicity of Gal in T-cell responses. The model of Gal+/+ organ transplantation into a Gal -/- SLA-matched recipient would be a valuable large animal model in the study of accommodation or B-cell tolerance.