The Re-Os isotopic systematics of spinel- and garnet-bearing mantle xenoliths from North and Central America, Europe, southern Africa, Asia, and the Pacific region were determined to define more precisely the isotopic composition of a hypothetical primitive upper mantle (PUM). When plotted against Al 2O 3 or other melt depletion indicators, the 187Os/ 188Os ratios of all xenolith suites display positive trends toward a uniform composition at a fertile mantle composition. A 187Os/ 188Os value for PUM of 0.1296 ± 0.0008 (level of confidence 95%) was defined on the basis of 117 spinel-bearing xenoliths from this work and data from the literature, including data for massif peridotites. The 187Os/ 188Os ratio of the PUM is similar to the range of compositions defined by ordinary and enstatite chondrites, not carbonaceous chondrites. Spinel-bearing mantle peridotites sampled by volcanism and peridotite massifs appear to have been extracted from a common fertile source (PUM) between 1 and 2 Ga ago. These peridotites now form part of the subcontinental lithospheric mantle underlying continental crust of similar or greater formation age.