A novel homogeneous enzyme immunoassay has been developed that takes advantage of the observation that the activity of a bacterial chitinase could be enhanced by the binding of a ligand conjugate of a monoclonal antibody to the enzyme (L-MAbchi). The enhancement was suppressed by an antiligand antibody (AbL). Free ligand molecules competed for AbL combining with L-MAbchi, and the suppression of the enhancement was reduced. Thus the concentration of free ligand could be estimated. Biotin was used as a ligand in this model experiment. When this monoclonal antibody was conjugated with a ligand (e.g., biotin), we furthermore found that the enhancement could be suppressed by an antiligand antibody (AbL). When free ligand molecules are added to the system, competition with L-MAbchi results to reduce the suppression of enhancement.