Abstract: An optical agglutination assay device for detecting cocaine. The device includes a hollow reaction cell defining a generally planar liquid receiving reaction chamber having a thickness which is insufficient to diminish the intensity of a ray of light passing through an aqueous reaction system in the chamber in a first direction transversely of the plane of the latter. The device also includes a pusher assembly for delivering an aqueous agglutination reaction system and an unknown substance suspected of containing cocaine into the chamber. The agglutination system is such that agglutination is inhibited in the presence of cocaine. The device also includes an optical transmitting and receiving unit aligned with the reaction cell for measuring the intensity of light reflected from the chamber as a measure of the occurrence of agglutination in the reaction system.