Abstract: A biological fluid from a first animal species (e.g. human serum) is assayed for an immunogen therein by mixing with the fluid (Ig-minus-Fc) fragments of an immunoglobulin from a second animal species (e.g. rabbit), the immunoglobulin being immunospecific to the immunogen or another component of the mixture whereby the immunogen can be determined. Interference can occur from reaction between first animal species antibodies and the said fragments, and in the method of the invention this interference is avoided or overcome, preferably by also including in the mixture different (Ig-minus-Fc) immunoglobulin fragments from the second animal species which react with said antibodies but not with said immunogen. These different fragments are preferably aggregated. The method of the invention is particularly applicable to particle agglutination assays.