1. Low-density (d 1.006–1.063g/ml) lipoproteins from normal human plasma were separated by differential preparative ultracentrifugation into six subfractions. Each low-density (LD) lipoprotein subfraction contained lipoprotein B as the major and lipoproteins A and C as the minor lipoprotein families. 2. Three lipoprotein B subfractions (LP-B), LP-B-III (d 1.019–1.030g/ml), LP-B-IV (d 1.030–1.040g/ml) and LP-B-V (d 1.040–1.053g/ml) were prepared from the corresponding LD lipoprotein subfractions by immunoprecipitating small amounts of lipoproteins A and C. 3. Determination of hydrodynamic properties indicated that LD lipoproteins consisted of three molecular segments characterized by a stepwise change in the molecular weight: LDL-I and LDL-II subfractions (d 1.006–1.019g/ml) with an average mol.wt. of 4.75X106, LDL-III (d 1.019–1.030g/ml) with a mol.wt. of 3.99X106, and LDL-IV, LDL-V and LDL-VI (d 1.030–1.063g/ml) with a mol.wt. of 2.85X106. 4. All three lipoprotein B subfractions had an average mol.wt. of 3.16X106. 5. The LDL-I and LDL-II subfractions consisted of lipoprotein B and lipoprotein C families which were present in the form of an association complex. This was isolated from serum by immunoprecipitation with antibodies to lipoprotein B. The complex had a mol.wt. of 4.35X106. 6. The results indicate a fundamental difference between the LD lipoprotein subfractions with d 1.006–1.019g/ml and those subfractions with d 1.030–1.063g/ml. In the former, lipoprotein B occurs as a part of an association complex, whereas in the latter it occurs as a separate entity.
