We have used a 2.2 kb fragment of the human apolipoprotein AI (apo AI) gene to screen a number of unrelated individuals for common restriction fragment length polymorphisms (RFLPs) of the gene. As well as the previously reported SstI RFLP (allele frequencies in normolipidaemic individuals 0.94 and 0.06) we have detected RFLPs with the enzymes PstI and XmnI (allele frequencies in normolipidaemic individuals 0.88 and 0.12 for both polymorphisms). In the population studied, the RFLPs appear to be in linkage equilibrium and can be used in conjunction as a haplotype, with a PIC value (polymorphism information content) of 0.5. Significant differences in allele frequency were observed between subgroups of hyperlipidaemic patients and normolipidaemic controls. There is no strong population association in our patient group between any allele of the RFLPs studied and hypertriglyceridaemia.