PROBLEM TO BE SOLVED: To provide an immunoassay reagent capable of expanding the measurable range to the high-concentration range of a measured material and to provide an immunoassay. SOLUTION: This immunoassay reagent contains thiocyanate suppressing the coagulation of latex grains in response to the concentration of a measured material so that the latex grain coagulation degree does not exceed the reaction limit absorbance in absorbance measurement. In this ammunoassay, a sample under test is brought into contact with a latex grain suspension carrying an antigen or an antibody for the measured material, then the latex grain coagulation degree after the contact is measured by absorbance. The thiocyanate is contained so that the latex grain coagulation degree does not exceed the reaction limit absorbance.
