Abstract 1. (a) Phagocytic cells produce oxygen radicals when exposed to opsonised bacteria or “foreign cells” generally. 2. (b) These oxygen radicals are an important part of the mechanism by which foreign particles are destroyed. 3. (c) Release of oxygen radicals and enzymes into the surrounding environment by phagocytic cells causes damage. I propose that chronic inflammatory disease, as illustrated by the autoimmune diseases, results when this damage gets out of hand. 4. (d) I further propose that excessive oxygen radical production due to environmental factors can initiate the autoimmune diseases.