Histological and immunofluorescence techniques showed that mononuclear cells invaded virus-infected foci in the livers of passively immunized mice within 10 hr of the receipt of immune spleen cells or hyperimmune serum; by 24 hr, marked destruction of virus antigens had occurred in these lesions. Immune cell transfer promoted denser packing of mononuclear cells in the foci and more efficient destruction of infectious material than immune serum. Similar liver lesions developed by the 6th day after sublethal, primary, subcutaneous infection in normal mice. In contrast, in mice with GVHR which were immunosuppressed but possessed hyperactive macrophages and unimpaired splenic interferon response, mononuclear cells did not invade liver lesions and the animals died. These results, together with data reported previously, indicated that mononuclear cell invasion of infected liver foci, triggered by CMI, was of key importance in recovery from primary mousepox. The roles of specifically sensitized lymphocytes and macrophages within lesions were not directly evaluated, but indirect evidence suggested that lymphocytes could cause no more than a halt in virus multiplication, and that macrophages were required for the inactivation of preformed virions. Possible augmentation of the efficiency of macrophages by locally-produced lymphocyte interferon, neutralizing antibody, or stimulation of their phagocytic and intracellular digestive capacity cannot be excluded
