Site Pg is an antigenic determinant which is present on the surface of serum lipoproteins and reacts with an antilpoprotein myelomatous immunoglobulin, the IgA, Ger. Firstly site Pg is extracted from LDL with the major part of lipids, by an ether-methanol mixture at 6 p. 100. Then the organic substrate obtained is evaporated under vacuum and dissolved in saline before being submitted to an extraction by ehter only. Under these conditions, site Pg remains in the aqueous phase. A polyacrylamide gel filtration on Biogel P2 allows then to separate the substance which reacts with IgA Ger. anti-Pg, from the aqueous phase. The identification of site Pg allowed us to recognize the presence of galactose, phosphorus and choline, and to evaluate its molecular weight of about 330. The reactivity of site Pg with IgA anti-Pg was measured by inhibition of passive hemagglutination and fluorescence quenching. The association constant of site Pg with the whole IgA Ger. or with its Fab fragment could thus be calculated (1.6 10(5) M-1 with the whole IgA; 2.3 10(5) M-1 with the Fab fragment). We showed also that if phosphorylcholine plays an immunodominant role, the presence of sugar seems necessary, since the association constant of the whole IgA Ger. with site Pg is higher than that of IgA Ger. with phosphorylcholine alone.