Cell mediated immune response in mice infected with influenza A (Scotland) 73 (H3 N2) was studied by defining of the leukocyte migration inhibition, in vitro lymphocyte transformation and phagocytosis of S. aureus 209 P by the peritoneal exudate cells. It was found that the infection induces several changes in the reactivity of lymphoid cells which react with influenza antigens. The macrophage migration-inhibition occurs and it sustains until the 14th day. The mitogenic response to phytohemagglutinin weakens but, in the early period of infection the phagocytosis does not change but on the 14th day it also weakens.