Abstract.Type III hyperlipoproteinaemia (HLP) is, amongst others, characterized by the E2/2phenotype as determined by isoelectric focusing of apolipoprotein E. However, one of our clinically symptomatic type III HLP patients showed a E3/3phenotype.After complexation with phospholipid vesicles, apo E from this patient was, in contrast with apo E from a type IV HLP patient (E3/4phenotype), unable to compete with low density lipoprotein (LDL) for binding to the specific LDL receptors on cultured human fibroblasts. This defect in binding to the LDL receptor was not due to an impaired lipid binding capability. The clinical symptomatic type III hyperlipoproteinaemia of our patient is probably due to a functionally inactive apo E3.