PURPOSE: To improve the detecting sensitivity for girdia lamblia(G.L.) in feces so as to diagnose the G.L. stably and surely by preparing a G.L.-specific antigen (GSA65) useful for diagnosing G.L. in feces and a single specific antibody to the antigen.

CONSTITUTION: At the time of preparing a refined GSA65, a partially refined mixture containing a GSA65 derived from the eluate of feces, the cyst of G.L. or its nutrient, and a contaminant is bonded to a matrix in which a monospecific polyclonal antibody is bonded to the GSA65 and recovered. The GSA65 has a molecular weight of about 65,000 and has a resistance to the resolution and digestion of proteins. After impurities are washed out from the matrix and the matrix is dried for about 30 to 60 minutes, a stable GSA65 Girdia Lamblia cyst precipiti is obtained by partially denaturing the complex by washing the matrix with an acidic water-alcohol mixture and a serum to GSA65 is obtained by introducing the precipiti to a manmmalian animal. When this serum is brought into contact with the eluate of feces, the generation of the complex can be detected under the presence of the G.L.