The aim of this study is to evaluate the effects exerted in vivo upon the general reactivity of the immune system by a commercial split influenza vaccine produced by the Cantacuzino Institute, Bucharest-Romania. The vaccine was intramuscularly administered to 14 volunteers with no precedent of influenza vaccination and no major immune disorders. We have investigated in vitro the polyclonal proliferation of peripheral lymphocytes and the phagocytosis developed by peripheral granulocytes, before and three weeks after vaccination. Our experimental results indicate that the vaccine might have a modulatory action on peripheral leukocyte concentration induces the activation of polyclonal lymphocyte proliferation and of the phagocytosis potential of granulocytes. These effects are not dependent on the age of the vaccinees.