Apolipoprotein B (apo B) levels were measured by radioimmunoassay in the serum and lipoproteins from normal subjects and patients with hyperlipoproteinaemia. The total serum apo B concentration in normal subjects was 0.91 +/- 0.16 g/l (mean +/- S.D.); in type IIa hyperlipoproteinaemia it was 2.24 +/- 0.61 g/l; in type IIb, 3.05 +/- 1.24 g/l; in type IV, 2.24 +/- 0.99 g/l; and in type V, 1.30 +/- 0.16 g/l. In normal subjects 5.6 +/- 2.1% (mean +/- S.D.) of total apo B was present in very low density lipoproteins (VLDL) and 93 +/- 9% in low density lipoproteins (LDL). Corresponding values for type IIa were 3.8 +/- 1.9% and 93 +/- 3%, for type IIb, 9.9 +/- 7.5% and 91 +/- 1%, for type IV, 16.9 +/- 9.5% and 81 +/- 9%, and for type V, 38.4 +/- 11.0% and 52 +/- 8%. The ratio of cholesterol to apo B in serum was decreased in types IIa, IIb and IV, and increased in type V whereas the ratio of triglyceride to apo B in serum was decreased in type IIa, normal in type IIb and increased in types IV and V. The ratio of cholesterol to apo B in VLDL was increased in types IIa, IIb and V, but normal in type IV, whereas in LDL, this ratio was normal in types IIa and V but reduced in types IIb and IV. The ratio of triglycerides to apo B in VLDL was normal in types IIa, IIb and IV but raised in type V. In LDL, this ratio was increased in types IIb and IV but normal in types IIa and V.