Anti-αGal IgM levels inGalT^−/−Cr2^−/−/GalT^−/− radiation chimeras that received nonmyeloablative conditioning with or without BMT from αGal+ BALB/c mice 8 weeks later. Serum after secondary nonmyeloablative conditioning of control and mixed chimeric (BALB/c secondary BMT donors) GalT^−/−Cr2^+/+ → GalT^−/−Cr2^+/+ (S+H+, groups 1 and 2), GalT^−/−Cr2^−/− → GalT^−/−Cr2^−/− (S−/H−, groups 3 and 4), GalT^−/−Cr2^−/− → GalT^−/−Cr2^+/+ (S+H−, groups 5 and 6), and GalT^−/−Cr2^+/+ → GalT^−/−Cr2^−/− (S−/H+, groups 7 and 8) radiation chimeras (where S denotes the Cr2 gene status of the nonhematopoietic stromal compartment and H denotes the Cr2 gene status of the hematopoietic compartment) was tested by ELISA assay 10 weeks after secondary conditioning. Significant titers of anti-αGal IgM were detected in conditioned S+/H+ and S+/H− GalT^−/− mice but were undetectable in conditioned S−/H− and S−/H+ GalT^−/− mice. *Statistically significant differences. There were 4 mice in group 1, 6 in group 2, 5 in group 3, 4 in group 4, 3 in group 5, 3 in group 6, 3 in group 7, and 3 in group 8. Data shown are mean ± SD.