Abstract: A reagent for the detection of rheumatoid factor in tests in tubes or microplates. The reagent contains a complex formed from red blood cells and from antibodies soluble with respect to these red blood cells, and immunologically fixed to the latter, said reagent being both stable and capable in given dilution intervals to give rise to hemagglutination only subject to contacting it with detectable concentrations of rheumatoid factor. The invention also relates to a process for making said reagent which comprises subjecting red cells and the antibodies to be fixed thereon to successive treatments with distinct aldehydes or tanning agents, as well as with proteinic substances, such as albumin, having dispersive properties.