A liquid, particularly but not exclusively a liquid of human origin such as blood serum or urine, is immunoas- sayed for antibodies, antigens or antibody: antigen complexes, using as a reagent in the analysis an active fraction from mouse ascitic fluid. This active fraction is a euglobulin and has the ability, like human C1q, to combine with antibody: antigen complexes but not with free antibody or antigen. Unlike human Clq, however, it remains active at high pH's and its activity is not destroyed by 0.1M putrescine or 0.1M hydrazine, so analyses on human body fluids can be carried out at high pH's or in the presence of putrescine and hydrazine, without interference from endogenous human Clq. The active fraction is a very broadly applicable reagent in immunoassays and particularly useful in techniques involving agglutination of latex particles.