We have determined the frequency of DNA polymorphism of the gene for human apolipoprotein B detected with XbaI in 525 Israeli offspring whose parents experienced a myocardial infarction. The relative frequencies of the X1 (8.6 kb) and X2 (5.0 kb) alleles were 0.67 and 0.33, respectively, with no significant differences between males and females and across the different origin groups. Significant variation in sex, age and body mass adjusted plasma levels of cholesterol (P = 0.02), LDL-C (P = 0.02) and apo B (P = 0.03) were associated with the XbaI polymorphism. An interaction with age was demonstrated. For young individuals a simple codominant association of the XbaI site with cholesterol and LDL-C was evident and the differences between the two homozygote groups ranged between 22 and 25 mg/dl. For individuals above age 25 these differences were about 12 mg/dl with no significant difference between the X1X2 and the X2X2 genotype groups. In our study sample the apo B XbaI polymorphism accounted for 1% of the variability of plasma cholesterol, LDL-C and apo B levels. The XbaI polymorphism also had an effect on the associations among lipid and lipoprotein variables. In conclusion, we have demonstrated an association of the apo B XbaI polymorphism with the metabolism of the apo B-containing lipoprotein particles in a sample of Israeli offspring with a family history of myocardial infarction.