Immunostimulating properties of the serum of patients suffering from myocardial infarction were demonstrated in experiments on albino mice and rats. Injection of the serum intensified antibody production in the animals, increased blood serum bactericidal activity in relation to E. coli and intensified lysozyme production and neutrophilic phagocytic activity. The properties listed should be linked with the accumulation in blood of patients with myocardial infarction of leucopoietins acting on the macrophages and granulocytes which are closely related to the mechanisms of natural resistance.