By way of gel double immunodiffusion a certain heterogeneity of low density lipoproteins was observed to manifest itself in an additional band of immunoprecipitation. The incidence of this additional band does not exceed 1/3 of the cases of low density lipoproteins studies in normal individuals, while in those with ischaemic heart disease the additional band is found twice-thrice as often, the highest incidence being noted in patients with ischaemic heart disease and Type IIb hyperlipoproteinemia (93% of cases). The revealed immunochemical heterogeneity of low density lipoproteins was shown to be not connected with the appearance of any new, additional antigen in their structure. Most probably it is attributable to the presence of lipoprotein particles with a quantitatively different protein and lipid composition, probably of intermediate lipoprotein metabolites in blood plasma, or conformation changes in the structure of low density lipoproteins.