An additive to be added to latex particles used in a reagent for a latex agglutination reaction, wherein the latex particles have not been subjected to blocking treatment, and the additive comprises a polymer containing more than 65% by mass and 99% by mass or less of hydrophilic repeating units (A) relative to all repeating units and 1% by mass or more and less than 35% by mass of hydrophobic repeating units (B) relative to all repeating units, and having a weight average molecular weight of 3,000 or more, characterized in that the repeating unit (A) contains at least a hydrophilic repeating unit represented by the following formula , wherein both of the hydrophilic repeating unit represented by formula and the hydrophilic repeating unit represented by formula may be contained, in formula , R 18 represents a hydrogen atom or a methyl group, and R 19 and R 20 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, in formula , R 21 represents a hydrogen atom or a methyl group, and R 22 and R 23 each independently represent an alkanediyl group having 1 to 3 carbon atoms; and the additive comprises one or more selected from the group consisting of a hydrophobic repeating unit represented by the following formula and a hydrophobic repeating unit represented by the following formula , as the repeating unit (B). wherein R 31 represents a hydrogen atom or a methyl group, R 32 represents -O-, ∗ -(C=O)-O-, ∗ represents a position bonded to a carbon atom to which R 31 in formula is bonded), and R 33 represents a hydrocarbon group. wherein R 35 represents a hydrogen atom or a methyl group, and Z represents a group represented by the following formula . X 1 represents -O- or -NH-, X 2 represents an organic group having a formyl group or a keto group and having 3 to 12 (preferably 4 to 10, more preferably 5 to 8) carbon atoms, and ∗ represents an atomic bond. the latex particles have not been subjected to blocking treatment, and the additive comprises a polymer containing more than 65% by mass and 99% by mass or less of hydrophilic repeating units (A) relative to all repeating units and 1% by mass or more and less than 35% by mass of hydrophobic repeating units (B) relative to all repeating units, and having a weight average molecular weight of 3,000 or more, R 18 represents a hydrogen atom or a methyl group, and R 19 and R 20 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, R 21 represents a hydrogen atom or a methyl group, and R 22 and R 23 each independently represent an alkanediyl group having 1 to 3 carbon atoms; and the additive comprises one or more selected from the group consisting of a hydrophobic repeating unit represented by the following formula and a hydrophobic repeating unit represented by the following formula , as the repeating unit (B). R 32 represents -O-, ∗ -(C=O)-O-, ∗ represents a position bonded to a carbon atom to which R 31 in formula is bonded), and R 33 represents a hydrocarbon group. X 2 represents an organic group having a formyl group or a keto group and having 3 to 12 (preferably 4 to 10, more preferably 5 to 8) carbon atoms, and ∗ represents an atomic bond. <EOS>