Measurement reagent latex particles, and the coated latex particles with a turbidimetric immunoassay measurement reagent

Object of the present invention is to provide an assay can be performed even in the assay reagent of high sensitivity with a thin latex particle measured when the concentration of test substance in the sample. The present invention provides an assay reagent latex particle, comprising: polymerizing a monomer mixture containing a monomer of the following (a) ~ (c) in an aqueous medium containing 7.5 to 25 wt% of a C1-4 alcohol in the copolymer surfactant obtained by copolymerizing not used. Wherein, (a) is a polymerizable monomer having a phenyl group, (, b) having a phenyl group and a sulfonate polymerizable monomer, (c) a polymerizable monomer having a naphthyl group.