(1) Red cells from acatalatic humans contain a small amount of catalase activity (0.1—1.3% of normal). By means of gel-filtration this material has been isolated and its properties compared with catalase from normal human red cells. Preparations from either source are identical in respect to (a) elution velocity on Sephadex G-100 column, (b) electrophoretic mobility on starch gel, (c) sensitivity towards azide and (d) precipitation in the Ouchterlony-test by anti-catalase serum. Based on these observations acatalasia is likely to be a "controller gene disease" rather than a structural gene defect. (2) The enzyme is lacking not only in red and white blood cells, but also in liver, skin, lymphoepithelial tissue and explanted fibroblasts. In contrast to East-Asian cases in Swiss acatalasia families no oral affections (gangraena) are observed and activity levels in the blood of heterozygotes vary to a much greater extent. (3) In loading test with formate (5 g orally in a single dose) normal and acatalatic subjects behave the same way. In either case formate is rapidly oxidized and only a few percent are excreted unchanged in urine. Based on similar experiments with 14C-formate in normal and germ-free rats this is considered as evidence that bacterial catalase effectively compensates the enzyme defect and therefore enables the acatalatic to perform oxidations due to peroxidatic catalase activity.