This paper is a brief review of studies which demonstrate that lymphocyte mediators can activate macrophages in vitro. Macrophages which have been incubated in lymphocyte mediator-rich Sephadex fractions show changed morphology, metabolism and function. These changes include an increase in adherence to glass. ruffled membrane movement, phagocytosis of some particles, glucose oxidation through the hexose monophosphate shunt and an increase in the activity of a membrane enzyme, adenylate cyclase. Such mediator-activated macrophages show enhanced bacteriostasis and tumoricidal activity. In addition, studies describing the role of membrane sugars and esterases in the interaction of migration inhibition factor and macrophages are reviewed.