Application of the proton condition of inorganic chemistry to the problem of describing the net acid content in a biological system. A "proton content" is defined that expresses the net acid content in terms of the detailed chemical composition of the system. This approach is compared with previous approaches to the quantitative description of acid-base metabolism and is then applied to the interpretation of experimental metabolic balances. The principal advantage of the mathematical formalism described in this study is that its use pinpoints the assumptions and makes it possible to handle unusual cases simply by including terms that are normally neglected. 53 pp. Refs.