Anti-H-2 transplantation sera were fractionated by electrophoresis (curtain electrophoresis and Pevikon block electrophoresis). The fractions were tested for their content of IgM, IgA, 7 Sγ1 and 7 Sγ2 immunoglobulins as well as for the following biological properties: hemagglutination, in vitro immunocytotoxicity, in vivo growth inhibition of allografted tumors, and in vivo enhancement-facilitation of allo-grafted tumors. IgM antibodies had little (if any) biological activity in hyperimmune transplantation sera. IgA antibodies appeared to be correlated with synergic hemagglutination, while direct hemagglutination was correlated with IgG (7 Sγ1 and 7 Sγ2). 7 Sγ2 antibodies were shown to be responsible for in vitro immunocytotoxicity against relevant antigen-bearing cells, as well as for in vivo specific growth inhibition of allotransplanted tumor, whenever the demonstration of an in vivo inhibition was made possible by the incidence of spontaneous takes. In vivo enhancement phenomenon was found to correlate with non-C'-fixing 7 Sγ1 (Arnason's IgA) and/or IgA (Arnason's IgY) antibodies, while there were suggestions that 7 Sγ2 at low concentration could play some role in enhancement.