(1) In two Swiss families 8 acatalatic and 30 hypocatalatic subjects have been found. Whereas blood catalase activity varies in hypocatalatic subjects from 16-85% of normal, a small but distinct residual activity has been found in acatalatics. This corresponds to 0.1-1.3% of normal activity and does not differ from blood catalase of normal subjects in respect to electrophoretic mobility and azide sensitivity. (2) All acatalatic subjects are in a good state of health. The analysis of their blood in respect to serum, red cell and white cell enzyme patterns revealed no or only minor differences, except that catalase is lacking in the white cells, too. In contrast to normal red cells, those from acatalatic subjects are unable to perform peroxidatic oxidation reactions in the presence of a suitable H202 donor. (3) Intact red cells from acatalatic humans - like those from the duck - exert an exceptionally high rate of methemoglobin formation when exposed to X-rays; it is 10-20 times higher than that in normal human red cells. Addition of small amounts of catalase before irradiation reduces the rate to that of normal.