The properties of the low-density lipoprotein (LDL) receptor were studied in skin fibroblasts from a homozygous familial hypercholesterolaemic subject, MM (MM cells), who exhibits a defect in the processing of the precursor form of the receptor. Despite the prolonged half-life of the precursor (3 h), essentially all was eventually processed to the mature form, which was degraded with a half-life of approximately 6 h. The receptor content of the MM cells, determined by radioimmunoassay, was slightly lower than normal and by immunoblotting it was estimated that 55% was present as the mature form at equilibrium. The mature receptor reached the cell surface and was internalized and recycled apparently normally, with about 60% accessible to pronase at any time. Immunoblotting, immunoassay and surface labelling with 125I all indicated that the cells contained twice as much surface receptor protein as would be expected from the binding of LDL. Maximum heparin-releasable LDL binding to MM cells was 20% of normal and the apparent affinity for LDL was reduced. The cells did not show the marked increase in affinity and reduction in binding exhibited by normal cells when cooled to 4 degrees C. Also, neither the apparent affinity of the receptors nor the maximum binding at 37 degrees C was greatly affected by an antibody that has been shown to reduce the affinity and halve the binding of normal cells. The results suggest that the mutation in the LDL receptor gene of these cells affected the ability of the receptors to bind LDL in a similar manner to the antibody, possibly by promoting aggregation of receptors on the cell surface, and halving the amount of LDL bound.