Tangier disease is a rare familial disorder characterized by enlarged orange tonsils, transient peripheral neuropathy, hepatosplenomegaly, and lymphadenopathy, as well as striking reductions in plasma high density lipoproteins (HDL) and their major protein constituents, apolipoproteins (apo)A-I and A-II. In order to test the hypothesis that Tangier patients have abnormal apoA-I or apoA-II, the in vitro lipoprotein binding and in vivo metabolic characteristics of these proteins isolated from normal and Tangier plasma, were studied in normal subjects and patients with Tangier disease. After incubation with normal plasma, significantly greater percentages of radiolabeled Tangier apoA-I were associated with the 1.063-g/ml supernate (6%) and the 1.21 g/ml infranate (19%), and a lower percentage with HDL (75%), than those observed for normal apoA-I (2, 8, and 90%, respectively). In contrast, the lipoprotein binding properties of normal and Tangier apoA-II were very similar. Following the injection of radiolabeled normal and Tangier apoA-I into normal subjects (n = 4), the mean residence times of the specific activity for apoA-I(Tangier) were significantly lower, both in plasma (1.29 d) and in HDL (1.34 d), than those observed for normal apoA-I (3.80 and 4.06 d). In Tangier homozygotes the decay rates of these tracers were very rapid and were similar. No significant differences between the kinetics of normal and Tangier apoA-II were observed in normal subjects (n = 2). Tangier homozygotes (n = 3) had mean plasma HDL cholesterol, apoA-I, and apoA-II concentrations that were 4, 2, and 11% of normal (n = 24), respectively, whereas for heterozygotes (n = 3) these values were 46, 62, and 68% of normal. In homozygotes, in contrast to normals or heterozygotes, a significant fraction of both apoA-I and apoA-II were found in the 1.063-g/ml supernate instead of in HDL. Homozygotes had apoA-I(Tangier) synthesis rates and residence times that were 41 and 5% of values observed for normal apoA-I in normal subjects, and for apoA-II in homozygotes, these parameters were 63 and 18% of normal. Heterozygotes had apoA-I synthesis rates and residence times that were 92 and 66% of normal, and for apoA-II these values were 101 and 64% of normal. These data are consistent with the concept that apoA-I(Tangier) is functionally and metabolically distinct from normal apoA-I, and is the cause of the striking hypercatabolism of apoA-I and apoA-II, and the lipoprotein abnormalities observed in Tangier disease.