The apoprotein B gene restriction fragment length polymorphism are studied using the endonuclease XbaI, and the pAB3.5C probe was studied in 128 healthy males aged 20-62 years (39.2 +/- 7.6). The genotypic prevalence was X1X1 26.6%; X1X2 47.7% and X2X2 25.7%. The allelic frequency was 50.3% X1 and 49.7 for X2. No differences in prevalence were observed related to age or body mass index. The genotype X2X2 was statistically associated with a 10% increase in total plasma cholesterol, LDL cholesterol and LDL Apo B levels (p less than 0.05). Up to 6% of the total plasma cholesterol levels were dependent on X2X2 genotype as shown by multivariate regression analysis. The X2X2 genotype may be a candidate marker in assessing increased risk for coronary heart disease.