A particulate latex for high-sensitive agglutination assay that barely poses non-specific reactions and can readily prepare diagnostic reagents, and a reagent for agglutination assay including the particle are provided. A particulate latex for agglutination assay, including a first polymerizable monomer having a phenyl group, a second polymerizable monomer having a phenyl group and a salt of sulfonic acid, and a third polymerizable monomer represented by Formula (1): CH2═CR1—COOCH2CH2O(PO2)OCH2CH2—N(CH3)3  (1) where R1 represents a hydrogen atom or a methyl group, wherein the density of functional groups in the third polymerizable monomer represented by Formula (1) on the surface of the particulate latex is 0.003 to 0.05 μmol/m2.