Abstract: An enzyme immunoassay for detecting an antigen in a biologic fluid or tissue which comprises contacting the fluid or tissue with an antibody specific for the antigen under binding conditions, at least one of the fluid or tissue and antibody having a solid component, contacting the resulting solid with a conjugate bindable with the antibody under binding conditions and determining the enzyme activity of the resulting solid phase is described. The conjugate is of peroxidase and an allergen, non-immunoglobulin protein or primary amino group containing drug having an average of 2-3 molecules of peroxidase per molecule of substance with an average molecular weight of about 30,000 daltons, prepared by reacting peroxidase previously treated with phenyl isothiocyanate and oxidized to form aldehyde groups with the substance to form a Schiff's base which is titrated with a reducing agent to form a stable conjugate.