Complement (C) inactivation by ammonia, ethylenediamine and methylamine in mouse serum was studied in relation to a possible adjuvant effect of the substances in a cell-mediated immune response. The amines caused a dose-dependent depletion of both alternative pathway (AP) and overall C activity in vitro and showed also pronounced adjuvant effects in the delayed type hypersensitivity response of mice to SRBC. A significant correlation between momentary inhibition of AP activity and adjuvanticity was observed (r = 0.9995; P approximately 0.02), suggesting a causative relationship between these two phenomena. Both effects seem to be a direct function of the number of amino-groups per molecule. Since, on the other hand, lysosomotropic activity of amines is known to decrease with the number of amino-residues, our findings exclude an important role of direct phagocyte inhibition in the immuno-adjuvanticity of these compounds. A longer persistence and improved presentation of antigen as indirect result of local C-depletion could account for the immunological adjuvant effects of amines.