The present invention provides a method and device for detecting the presence of binding ligands, especially blood group antigens or antibodies thereto, which utilize a matrix of substantially noncompressable microparticles, which matrix provides superior performance in allowing movement of nonagglutinated reactants, especially red blood cells, while constraining, preferably in a so-called "band formation" agglutinated reactants, especially red blood cells. The device of the invention further comprises a chamber the transverse cross-sectional shape thereof being an ellipse, or preferably a flattened ellipse with two sides parallel to the longitudinal axis.