The conditions were optimized for the recovery of F(ab')2- and Fab'-fragments from monoclonal antibodies produced by the HSV.A.3.3 clone and production on their basis of peroxidase conjugates by periodate and maleimide methods to be used in the enzyme immunoassays. The immunoglobulins isolated from the monoclone HSV.A.3.3 were found to belong to the subclass IgG1. The conditions were chosen to derive a homogeneous F(ab')2-fragment from intact IgG by means of restricted proteolysis and purification on protein A-sepharose CL-4B. A highly sensitive test system was developed for diagnosis of herpes simplex virus antigens using F(ab')2- and Fab'-fragments as sorbents and conjugates. The sensitivity of the system was 0.2 ng virus protein.