SUMMARY Antibody response of A.CA, A.SW, and C57B1 mice on immunization with A/Sn tissue was studied. Gel filtration on Sephadex G200 revealed two antibody classes: an earlier 19S (peak titers day 4-5) and a later 7S (peak titers day 12-15). On repeated immunization A.CA and A.SW responded with 7S antibody, C57BL with 19S and 7S. Some strain differences in antibody pattern were found after hyperimmunization with sheep erythrocytes, 19S and 7S activities were studied in vitro. In cytotoxic and hemolytic assays orders of reaction close to 1 were found for 19S and close to 2 for 7S. 7S gave positive staining in indirect fluorescent tests whereas 19S was not detectable. The findings are discussed in relation to efficiency per molecule in the antibody classes. Parallellism between strain differences in antibody pattern and homograft reactivity is pointed out.