Apolipoprotein B (Apo B) exists in different types of particles in human plasma: Lp B containing only Apo B, Lp B:E containing Apo B and Apo E, Lp B:C-III containing Apo B and Apo C-III, Lp B: (a) containing Apo B and Apo (a) and so on. The physicochemically defined lipoproteins were found to be heterogeneous with respect to this concept. A particle such as Lp B, for example, may occur in any segment of the density spectrum depending on the composition and content of its lipid complement. These particles are metabolically distinct and their quantification is essential for better understanding of lipid transport disorders. Using new immunological procedures, we have identified some subpopulations of Apo B containing lipoproteins which are more abondant in atherosclerotic patients and which characterize some dyslipoproteinemic states. Drugs decreasing Apo B act differently on these different types of particles. The results presented substantiate the usefulness of the study of lipoprotein particles defined by their apolipoprotein composition for future clinical, pharmacological and epidemiological studies.