Various immunologic properties of 19 S and 7 S isoantibodies directed against mouse H-2 isoantigens were investigated in vitro and in vivo. 19 S antibodies prepared in strains A.SW, A.BY, C57BL and B10.5M against strain A were more efficiently hemolytic and cytotoxic than 7 S antibodies, but less potent in agglutination tests. Antisera prepared in strain A.CA were exceptional, however, since 19 S and 7 S antibodies were equally efficient in hemolytic, cytotoxic and agglutination tests. The number of lysed red cells or killed nucleated cells decreased slowly with increasing dilutions of 19 S antibodies, whereas there was a rapid decrease with 7 S antibodies. 7 S antibodies were more efficient in eliminating H-2 incompatible red cells than 19 S antibodies in spite of equal agglutinin titers, but were less competent to induce the change to hemolytic resistance characteristic for mouse red cells after contact with antibodies in vivo. Passive transfer of 7 S antibodies induced immunologic enhancement of tumor homografts and inhibited the secondary immune response, whereas 19 S antibodies had no detectable effect in any of the tests. It is suggested that these results are caused by quantitative differences between 19 S and 7 S antibodies. The 19 S antibodies are presumably more efficient on a molecular basis in various serologic tests in vitro, but less competent in tests requiring the participation of a minimum number of antibody molecules, such as immunologic enhancement and inhibition of the immune response.