There is provided a particle-enhanced immunoassay method for cystatin C in a human body fluid, which has higher specificity and is easily automatable at low cost as compared with conventional assay methods that use large amounts of polyclonal antibodies having low specificity or monoclonal antibodies having high specificity but having poor agglutinability.Disclosed is an assay method specific to cystatin C in a human body fluid, which is a particle-enhanced immunoassay method utilizes a combination of antibody-coated particles in which some insoluble carrier particles are coated with anti-human cystatin C monoclonal antibodies of one type which have high affinity and in which some other insoluble carrier particles are coated with anti-human cystatin C monoclonal antibodies of one type which recognize an epitope different from that of the monoclonal antibody of the first-mentioned type and which have relatively low affinity, wherein the binding amount of the anti-human cystatin C monoclonal antibodies relative to the total weight of the particles being coated with the corresponding antibodies is less than 5% by weight.