Abstract: The neutralization of F (ab')₂ fragment of anti-HCG-IgG with HCG was investigated using mouse uterine weight assay and uterine histological examination. No significant increase in uterine weight and endometrial hyperplasia were observed in low dilution of F (ab')₂ fragment of anti-HCG-IgG. The endometrial hyperplsia was not found on histological section, but in relatively high dilution, a serious uterine edema and hyperemia with infiltration of phagocytes occurred. These investigations show that the Fc portion of anti-HCG is not roquired in the neutralization with HCG.