The gene for adrenoleukodystrophy (X-ALD), a peroxisomal disease characterized by excessive accumulation of very long-chain (VLC) fatty acids (> C22:0), has recently been identified by positional cloning, and it is predicted to encode a protein (ALD-P) of 745 amino acids [(1993) Nature 361, 726]. Using Western blot analysis of subcellular organelles purified by isopycnic density gradient centrifugation from X-ALD and control fibroblasts, we show that the monoclonal antibodies directed against ALD-P cross-react with a 75 kDa protein in intact peroxisomes and that ALD-P is an integral component of the peroxisomal membrane. Moreover, no signal for ALD-P was detected in peroxisomes from X-ALD patients with deletion of the ALD gene.