C3H mice injected with spleen cells from C57L donors sensitized against C3H tissue developed greater splenomegaly but neither accelerated nor intensified skin allograft immunity as compared with recipients of nonsensitized cells. Recipients of sensitized cells developed lesser splenic plasmocytic responses. Ferrokinetic changes and focal cellular infiltrations of the liver characteristic of graft-versus-host activity were observed in mice injected with sensitized allogeneic spleen cells. These findings are interpreted as supporting the view that the greater splenomegaly in recipients of sensitized cells is due to graft-versus-host activity rather than greater antigenic stimulation of the recipient.