A polymorphism of the apolipoprotein AII gene (on chromosome 1) was investigated using genomic hybridisation analysis. The two common alleles at this locus were defined by MspI restriction fragments of 3.0 kilobase pairs (M3.0) and 3.7 kilobase pairs (M3.7) respectively. The M3.7 allele was significantly more common (P less than 0.02) in Caucasian subjects who were normo-lipaemic (34%, 20/59) than in those who were hypertriglyceridaemic (16%, 16/98). Serum triglyceride levels were measured in 126 Caucasian subjects with different combinations of disease-associated alleles at the ApoAII and ApoCIII gene loci. Mean serum triglyceride levels were found to be significantly higher (P less than 0.05) in subjects with disease-associated alleles of both the ApoCIII and ApoAII genes, compared with subjects with a disease-associated allele of one or neither locus.