A method for immunochemical determination of a hapten in a sample is disclosed, in which(A) a high-molecular compound to which the hapten is bound (reagent A),(B) insoluble carrier particles carrying thereon an antibody to the hapten (reagent B), and(C) magnetic substance-containing insoluble carrier particles carrying thereon an antibody to an antigenic determinant in the high-molecular compound and different from the hapten (reagent C),are used. These three reagents are dispersed in the sample, then, a magnetic field is applied to separate from the reaction mixture unreacted reagent (C) and agglutinated particles formed from the reagent (B) and the reagent (C) through the reagent (A). The amount of the reagent (B) remaining dispersed in the reaction mixture is measured, thereby determining the extent of competitive inhibition to the agglutination of the reagent (B) and the reagent (C) through the reagent (A) by the reaction between the hapten in the sample and the reagent (B). The method of the present invention requires no troublesome operations such as separation-washing and is free from any problem is safety. By the method of the present invention, the concentration of a hapten in a sample is determined by simple operations in a short time and with high sensitivity.