In 1979, Steinberg and colleagues described a unique kindred with familial hypobetalipoproteinemia (Steinberg, D., Grundy, S. M., Mok, H. Y. I., Turner, J. D., Weinstein, D. B., Brown, W. V., and Albers, J. J. (1979) J. Clin. Invest. 64, 292-301). Recently, we demonstrated the existence of an abnormal species of apolipoprotein (apo-) B, apo-B37 (Mr = 203,000) in nine members of that kindred (Young, S. G., Bertics, S. J., Curtiss, L. K., and Witztum, J. L. (1987) J. Clin. Invest. 79, 1831-1841; Young, S. G., Bertics, S. J., Curtiss, L. K., Dubois, B. W., and Witztum, J. L. (1987) J. Clin. Invest. 79, 1842-1851). Apolipoprotein B37 contains only the amino-terminal portion of apo-B100. In affected individuals most of the apo-B37 is contained in the high density lipoprotein (HDL) fraction (d = 1.063-1.21 g/ml), where it is the principal apolipoprotein in a unique lipoprotein (Lp) particle, Lp-B37, which contains little, if any, apo-A-I. However, the most abundant lipoprotein in the HDL density fraction is a smaller particle, which contains apo-A-I, but no apo-B. The Lp-B37 particles were isolated from the HDL of affected individuals by immunoabsorption of apo-B37. Selected affinity antibodies specific for apo-B37 were used to prepare an anti-apo-B37-Sepharose 4B column. Lipoproteins not bound by the column (unbound HDL fraction) contained apo-A-I, but no apo-B. The Lp-B37, which was eluted from the column with 3 M KI, contained apo-B37 and trace amounts of apo-A-I, but no apo-B100. Over a 4-h period, normal human fibroblasts degraded 10-fold more 125I-low density lipoprotein (LDL) than 125I-Lp-B37. Also, whereas addition of excess unlabeled LDL markedly reduced degradation of 125I-LDL, it did not significantly reduce the degradation of 125I-Lp-B37. Unlabeled Lp-B37 did not inhibit uptake and degradation of 125I-LDL by fibroblasts. These data suggest that the amino-terminal portion of apo-B100, when expressed on a naturally occurring lipoprotein particle, does not contain a functional apo-B,E(LDL) receptor binding domain.