We report the detection of low-density lipoprotein (LDL) receptors by the technique of receptor blotting in fibroblasts from a patient with homozygous familial hypercholesterolemia (FHC) previously classified as "receptor negative." Solubilized receptors were electrophoresed, transferred to nitrocellulose paper, treated with LDL followed by radiolabeled antibody to LDL, and visualized by autoradiography. GM 2000 FHC fibroblasts revealed LDL receptors with an apparent molecular weight of approximately 140,000, the same as in normal cells. LDL receptor activity by blotting in GM 2000 cells was greatly diminished in comparison with normal cells, but was calcium dependent. Receptor activity was also detectable by conventional monolayer binding and degradation assays. Thus, GM 2000 cells have profoundly diminished LDL receptor activity, but retain the genetic capacity to make LDL receptor material of normal molecular weight that is capable of binding LDL. Previous studies have demonstrated the presence of trace amounts of immunoreactive LDL receptor protein in fibroblasts from some receptor-negative FHC homozygotes. Our work extends these studies by demonstrating the ability of this material to bind LDL.