Summary: Small quantities of ammonia or amines inactivate complement by destroying a relatively heat stable component, not identical with the third component. A fourth component of complement is therefore postulated. The action of ammonia is quite specific, for other alkalies have no similar effect. The possible significance of this new component in explaining difficulties and discrepancies in previous work on complement is discussed. The relatively heat-stable components of serum are much more stable towards ultra-violet light than the heat-labile components.