An anti-Fas ligand antibody capable of suppressing Fas ligand-induced apoptosis to a high degree, especially humanized antibodies having less antigenecity to human, and a method for assaying the Fas ligand in human body fluid utilizing such an antibody.Measurement of the Fas ligand in human body fluid would be useful for elucidation of hepatitis B, hepatitis C, HIV infections and the like, and for prediction and diagnosis of pathological conditions associated with such diseases.