PURPOSE: To obtain a serological diagnostic reagent for syphilis that is easy to handle and reacts rapidly as well as be able to evaluate the result as positive or negative with naked eye, by treating a lipid antigen with sudan black followed by sensitizing with kaolin.

CONSTITUTION: Treatment of a lipid antigen with sudan black is performed so as to dissolve a suitable amount of sudan black in a alcoholic solution of a lipid antigen. This mixed solution of the sudan black and the lipid antigen is mixed with kaolin in a physiological salt solution or a buffer solution, and warmed for a suitable period at room temperature W 100°C, so the colored lipid antigen is absorbed in kaolin and sensitized. The sensitized kaolin is collected and allowed to suspend in a suitable buffer solution, and, if desired, EDTA, choline chloride, etc. are added thereto, then diluted with purified water to the optimum concentration in consideration of absorbance and reaction sensitivity to a standard specimen of Wassermann's antibody. According to this procedure, the results are evaluated with naked eye without using a microscope.