PURPOSE: To prevent such trouble that accurate quantitative analysis becomes imposible because an anti-human serum is modified by shaking or stirring during transport to generate turbidity which is then added to the turbidity generated by antigen-antibody reaction, by adding a surfactant to said anti-serum reagent.

CONSTITUTION: A surfactant selected from a nonionic surfactant such as a polyoxyalkylene condensate or fatty ester of plyhydric alcohol, an anionic surfactant such as a phosphoric ester, an amphoteric surfactant such as a N-alkylbetain type and a cationic surfactant such as quaternary ammonium salt and fitted to an anti-serum reagent is added to the anti-serum itself, one diluted with a diluent for the anti-serum reagent or the conditioned anti-serum reagent due to the addition treatment of polyethylene glycol in proper concn. By this method, the generation of turbidity is prevented so that the turbidity generated by shaking or stirring the anti-serum reagent during transport is not added to the turbidity due to the flocculation generated by antigen-antibody reaction.