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Passively administered alloantiserum suppressed the in vivo growth of allografted mouse plasmacytoma X5563 with the aid of host cells, probably macrophages, but not with complement. Furthermore, it was shown that alloantiscrum probably participates in the process of tumour cell destruction in vivo as opsonin rather than as rytophilic antibody. On the other hand, administration of a decreased dose of the same whole antiserum or its fractions of γM as well as γG, or prior active immunization with living tumour cells, could induce the characteristic pattern of immunological enhancement. From concurrent studies, it was considered that enhancement might have been induced by the suppression of the development of the host's own immunity against tumour. [ABSTRACT FROM AUTHOR]