Whole rock analyses and selected trace element data are reported for nineteen, extremely fresh, peridotite xenoliths, together with the host ankaramite and associated carbonatite tuff, sampled from the Lashaine volcano in northern Tanzania. For comparison, data for a garnet peridotite from Matsoku in eastern Lesotho are included. These xenoliths are inferred to be derived from the garnet peridotite zone of the upper mantle from depths of about 150 km. The chemical data indicate that, despite considerable compositional heterogeneity, two distinct groups of peridotite inclusions have been sampled. One group, the garnet peridotites, are distinguished from the other group, the garnet-free peridotites, by higher concentrations of Al 2O 3 (> 1%) and lower amounts of MgO (< 46%). Other, less pronounced, differences include a tendency for higher concentrations of SiO 2, CaO, Na 2O, K 2O, Cr 2O 3, Rb, Y, Zr and Nb, and higher Rb Sr ratios in the garnet peridotites than in the garnet-free rocks. Evidence is presented which indicates that the peridotite inclusions are accidental, and are genetically unrelated to the ankaramite host. It is also shown that in contrast with similar inclusions in kimberlite, most of the Lashaine inclusions have not been substantially contaminated by the host rocks. Consequently, the compositions of these inclusions are pertinent to questions concerning the composition of the upper mantle. In comparison with available data for peridotite inclusions and inferred mantle compositions, the Lashaine inclusions appear to be derived from residual mantle material. However, the K and Rb concentrations are at variance with this interpretation, since they are substantially higher than is typical for most uncontaminated peridotite inclusions, and would necessitate unacceptably high concentrations of these elements in the primary, undepleted material. Two interpretations are advanced: either the Lashaine garnet peridotite inclusions are themselves samples of relatively undepleted primary mantle material, or they are samples of depleted residual material that has been contaminated at or close to the source, prior to incorporation in the ankaramite magma.