The present paper describes a study of the kinetics and mechanism of the heterocoagulation of two types of sensitized latexes in the presence of human chorionic gonadotropin (HCG). Two identical polystyrene latexes bearing carboxylic acid surface groups were sensitized by covalent coupling of monoclonal antibodies specific for the alpha HCG determinant on one type of latex and for the beta HCG determinant on the other type. Aggregation was initiated by addition of HCG to mixtures of the two sensitized latexes. Using the Coulter counter method, the aggregate size distribution c(n) was determined as a function of the number n of elementary constituents, thus enabling calculation of the number N(t) and weight S(t) average sizes of the aggregates. In one series of experiments, HCG was initially fixed only to one sensitized latex, the other remaining free of HCG. In a second series, the three components were mixed simultaneously. Comparison of the aggregation rates showed preferential recognition of HCG on the latex bearing the monoclonal antibody specific for the beta HCG determinant, while the variations of the intrinsic reactivities with HCG concentration were attributed to a cooperative mechanism.