The presence of even a small extent of agglutination among erythrocytes in aqueous suspension can be ascertained by a discrete-particle light-scattering technique. This provides a means of mechanizing a wide variety of assays done in connection with blood-banking, as well as other procedures that rely upon the agglutination of sensitized particles or cells. We describe an agglutination-detection system designed to serve this function, and demonstrate its sensitivity.