Products of activated lymphocytes are known to modulate the function of macrophages (1-3). Migration inhibitory factor (MIF) 1 is one of the best studied mediators of cellular immunity (4). Although many investigators have tried to analyze the biological and physicochemical properties of MIF from different species, the active substance has not been purified to homogeneity. MIF produced by guinea pig lymphocytes appears to be a glycoprotein with a mol wt of 35,000-55,000 daltons (5). Rocklin et al. (6) were able to differentiate highly purified guinea pig MIF from partially purified human MIF by using gel filtration, disc electrophoresis, isopycnic centrifugation, and enzymatic treatment. They determined MIF activity after Sephadex gel filtration in fractions containing molecules with a tool wt of 23,000 daltons. The production of antibodies to guinea pig and human lymphokines was demonstrated by Yoshida et al. (7) and Bendtzen (8), and Geczy et al. (9) produced antibodies to partially purified guinea pig MIF, which inhibited the delayed skin reaction of sensitized guinea pigs challenged with purified protein derivative. These results prompted us to initiate immunochemical studies of the mediators of cellular immunity in man. In view of the role of MIF in cellular immunity, the isolation and characterization of human MIF is of paramount importance for the full evaluation of its role in different diseases, and for the development of rapid qualitative and quantitative methods for its detection. This paper describes the purification and immunochemical characterization of human MIF produced by concanavalin A (Con-A)-activated lymphocytes. The specificity of anti-MIF antibodies is demonstrated by using sensitive immunoelectrophoretic techniques. MIF activity on guinea pig peritoneal macrophages, as measured by the capillary test, is correlated with its effect on the electrophoretic mobility of macrophages.