We determined reference values of apolipoproteins A-I (apo A-I) and B (apo B) in serum from a population of 448 healthy subjects (265 men and 183 women, ages 18 to 61 years) by a kinetic immunonephelometric procedure. Frequency distributions of apo A-I were normal, whereas those of apo B were not and yielded asymmetrical curves. Thus, reference intervals for apo A-I were determined as mean +/- 2SD (1.08-1.89 g/L), but a nonparametric method was used for determining reference intervals for apo B (0.60-1.94 g/L). Apo B concentrations were significantly higher (P less than 0.001) in men than in women (0.63-2.01 g/L, mean 1.21 g/L; and 0.54-1.91 g/L, mean 1.08 g/L, respectively). No significant differences for apo A-I between men and women were observed. Concentrations of both proteins increased with age, but apo B increased more than apo A-I. We conclude that not only sex but also the age of the subjects must be considered in interpreting laboratory results for apolipoproteins.