Age-dependent decreases in the protein concentrations of the nucleocytoplasmic transport factors karyopherin alpha2, CAS, and RanBP1 were found by comparing fibroblast cultures obtained from young, mature, and old human donors. Karyopherin 0 1 levels do not change with age and present very little variation among donors. The decrease in the concentration of transport factors is accompanied by a reduction in the protein import rate in fibroblasts from old donors, as detected by a change in the intracellular localization of a test transport substrate that shuttles between the cytoplasm and the nucleus. Measurements of concentrations of the same import factors in organs and tissues of old mice revealed a decrease of CAS in kidney, lung, and spleen. The import reduction in old age is expected to lead to impaired activity of proteins whose functions depend on timely import into the nuclei. (C) 2002 Elsevier Science (USA). All rights reserved.