The properties of five monoclonal antibodies (McAbs) which react with free kappa chains but not with IgG kappa, IgM kappa or IgA kappa have been studied. IgG preparations stored for long periods sometimes contained low and high molecular weight material reacting as free kappa. This could be removed by passing the preparation through an affinity column to which two of the McAbs had been coupled, but reactive material reappeared when the purified IgG was heated at 65 degrees C. Two of the McAbs from one fusion reacted with F(ab')2 fragments of IgG whereas two from another fusion did not. Pairs of McAbs (one from each fusion) were much more sensitive in the detection of free kappa, particularly monomer, than a single McAb. This provided the basis for a very sensitive method of detection of free kappa chains in human body fluids and culture supernates.