In 43 patients with acquired and congenital cardiac defects and in 4 others with ischemic heart disease some indices of immunological reactivity were studied before and after operations performed under extracorporeal circulation. In patients with cardiac defects and ischemic heart disease, in whom circulatory disorders were being observed for a long time and the cellular and humoral factors of the immunological reactivity of the organism were at a low level, a reduced capability of the segmentonuclear neutrophils in the peripheral blood to undergo phagocytos is along with the bactericidal properties of the serum and skin were demonstrable. With the post-operative period not complicated by an infection the normalization of these indices occur in a week's time. A moderately increased phagocytosis or, conversely, the lack of any tendency towards normalization during the 1st post-operative week is an unfavourable sign and, as a rule, is indicative of a developing infection-induced complication.