A simple, fast method for the measurement of serum alpha 1-antichymotrypsin by an immunoturbidimetric technique is described. The assay spans the range 0.21 g/L to 2.83 g/L which covers adequately the majority of concentrations encountered in normal and pathological sera. The precision is good and the accuracy acceptable, as judged by a comparison with an electroimmunoassay technique. A reference range has been obtained using samples collected from blood donors.