Leucocyte-mediated phagocytosis and antibody-dependent cellular cytotoxicity (ADCC) were tested in 48 children with type-1 diabetes and in 22 healthy children. Both phagocytosis and ADCC for opsonized 51Cr-erythrocytes significantly decreased in the diabetics. Phagocytosis decreased in well and in poorly balanced diabetics, but the latter, having type-1 diabetes for less than 5 years, exhibited a lower phagocytic capacity than the patients with a longer duration of disease. The decrease of ADCC in poorly balanced patients was greater than in the well balanced ones as compared to the controls. The duration of diabetes was without influence on their leucocytes' ADCC.