Abstract: Immunoassays for antigens or haptens are effected using, instead of an immunoglobulin antibody, the F(ab').sub.2 fragments thereof. In this way, interference from endogenous RF and Clq in the fluid under assay is avoided without the necessity of pre-treating the fluid to remove or inactivate the RF or Clq. The assays may be of various types including latex agglutination and competitive binding assays. A reagent for such use comprising F(ab').sub.2 fragments insolubilized on a water-insoluble substrate, especially a particulate substrate, is described.

Abstract: Insolublized antibodies, F(ab').sub.2 fragments thereof and protein antigens are made by coupling to a water-insoluble substrate using a bridging agent which covalently binds to sulphur atoms in antibody, fragment or protein rather than to free amino groups therein. This is preferably effected by using a chloracetyl-terminated bridging agent. Immunoassays utilizing these insolubilized reagents are described. One novel class of bridging agents comprising anhydride and monochloracetyl end groups, which is useful for making the said reagents, is also disclosed.