In recent years much has been learned about the essential role of peroxisomes in cellular metabolism. Much less, however, is known about the permeability properties of peroxisomes although it is well established now that peroxisomes are impermeable to small molecules which implies the existence of transporters in the peroxisomal membrane. In this paper we report the identification of PMP34, a peroxisomal membrane protein belonging to the mitochondrial solute carrier family, as an adenine nucleotide transporter. This is concluded from different experimental findings including rescue of the defect in medium-chain fatty acid oxidation in Saccharomyces cerevisiae cells in which the ANT1 gene coding for Ant1p, the peroxisomal adenine nucleotide carrier, was disrupted. Furthermore, we have purified PMP34, reconstituted the protein in proteoliposomes, and provide direct proof that PMP34 is an adenine nucleotide transporter. (C) 2002 Elsevier Science (USA). All rights reserved.