Circulating seric immunocomplexes have a variable significance in every case and small amounts can be founded in nornal people. Sometimes immunocomplexes are a defensive mechanism against chronic antigenemia, but in other cases they produce important tissue injury, as in glomerulonephritis. There are many tests for detecting them, but no single test detects all types of complexes because of their heterogeneity. Authors explain the polyehthilene-glycol precipitation test which is compared it to other methods, as binding to C'1q, Fc or C'3b receptors or physical separation tests. Polyethilene-glycol precipitation was performed in several concentrations of albumin-antialbumin complexes. When eight sera were heated to 60 degrees C during 30 minutes the levels of precipitate decreased and they suggest it is due to antigen and antibody dissociation. Inversely, IgG progressive concentrations increased by formation of heat aggregated immunoglobulins. The 10 times repeated freezing and thawing of sera increased the optical density of precipitates from 0.047 to 0.057 and the raising was higher in pathological than in normal sera. Although the repeated thawing must be avoid its' role does not seem be very relevant.