Lysosomes are late-endocytic organelles which primarily contribute to degradation and recycling of cellular material. From a previous proteomics study of purified rat liver lysosomal membranes we identified a protein from the Arf-family of small GTPases, Ar18b. Although proteins of the Arf-family have roles in a wide range of cellular functions, most notably roles in protein/vesicular trafficking, Ar18b represents the first from this protein family to be associated with a late-endocytic organelle. We demonstrate the co-localization of this protein with various lysosomal markers in different cell lines by confocal-immunofluorescence microscopy. We also show that GTP-restricted mutant Ar18b localizes to lysosomes and causes their redistribution to the periphery of the cell and into membrane projections. This indicates that Ar18b is involved in trafficking processes for lysosomes. (c) 2006 Elsevier Inc. All rights reserved.