Native and modified with ammonium ions alpha(2)-macroglobulin forms exhibit similar abilities to influence proliferative activity and secretion of immunoglobulins in cultures of human blood mononuclear cells in vitro. Effects of modified with ammonium ions alpha(2)-macroglobulin were more significant than those of native alpha(2)-macroglobulin. Revealed properties of native alpha(2)-macroglobulin could be conditioned by a partial modification of its structure with endogenous proteinases or cytokines, some of which were shown to be common affinants for both forms.