When insoluble magnetic carrier particles are used in immunoassay, a method is provided which is suitable for saving labor and for treating a large number of samples within a short time while avoiding problems including difficulties in the stability of sensitized insoluble magnetic particles and in their preparation. In assaying an antigenic substance in test samples, no use is made of insoluble magnetic particles carrying an antibody specific to the said antigenic substance but insoluble magnetic carrier particles are provided in a state free from adsorbing said antibody, etc. Then the antigenic substance per se to be assayed is adsorbed on the insoluble magnetic carrier particles followed by reaction with a labeled antibody specific to the said adsorbed antigenic substance. Thus, the antigenic substance in the test sample can be efficiently assayed in a mode suitable for automation.