Spleen cells from primed mice but not from normal mice had the capacity to adoptively transfer an anamnestic antitoxin titer in irradiated animals in the absence of transplanted bone marrow cells, and during retarded myeloid regeneration.

Spleen cells alone or bone marrow cells alone produced an insignificant and a moderate peritoneal eosinophil response, respectively, to antigen.

In the presence of bone marrow cells, normal spleen cells augment the capacity of recipient animals to mount an eosinophil response to antigen. A much greater augmentation occurs in animals reconstituted with splenic or thymic cells from primed animals.

The increase in antitoxin titers appears to be independent of the response of eosinophils since: (a) marked accumulation of eosinophils can occur in animals with no measurable humoral antitoxin, and (b) high antitoxin titers can occur in animals which do not have marked eosinophil responses.

It is suggested that a thymic-derived or thymic-dependent mononuclear cell population is necessary for optimal eosinophil response to antigen. The neutrophil and mononuclear cell responses to antigen are determined by different mechanisms from those which determine the eosinophil response. These studies together with earlier findings strongly indicate that the eosinophil granulocytes play a role in the immune response to antigen.