Concanavalin A (con A) binds specifically to alpha-D-glucosyl, alpha-D-mannosyl and sterically related structures. Many serum glycoproteins contain such residues and will therefore bind and precipitate con A. Using this principle, a nephelometric method was developed for measuring total con A binding proteins in serum. The effect of parameters such as con A to glycoprotein ratio, polymer enhancement, reaction time, stability of the complexes and potential inhibitory molecules such as glucose and calcium were assessed. A brief clinical evaluation of the assay in human sera showed that con A binding correlates well with specific acute phase protein-changes in inflammatory disease and may be used as an index of disease activity. Preliminary studies show that con A binding also reflects disease activity in a number of animal species and thus may provdie a much needed index of inflammation in the experimental animal and in veterinary practice, where species specific antisera to acute phase proteins are not widely available and protein analyses have been rather neglected. The assay is rapid, cheap and may be performed on automated nephelometric systems.