We present whole rock 187Os/188Os data for the most mafic lavas along the Lesser Antilles arc (MgO = 5-17wt.%) and for the subducting basalt and sediments. 187Os/188Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between 187Os/188Os and Os concentrations and between 187Os/188Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed. 87Sr/86Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with 187Os/188Os. The assimilant is inferred to have a MORB-like 87Sr/86Sr with high Sr (>700ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ~2.5) and 187Os/188Os>0.2. Such compositional features are likely to correspond to a plagioclase-rich early-arc cumulate. Given that assimilation affects lavas that were last stored at more than 5kbar, assimilation must occur in the middle-lower crust.Only a high MgO picrite from Grenada escaped obvious assimilation (MgO=17%wt.%) and could reflect mantle source composition. It has a very radiogenic 87Sr/86Sr (0.705) but a 187Os/188Os ratio that overlaps the mantle range (0.127). 187Os/188Os and 87Sr/88Sr ratios of the sediments and an altered basalt from the subducting slab vary from 0.18 to 3.52 and 0.708 to 0.714. We therefore suggest that, unlike Sr, no Os from the slab was transferred to the parental magmas. Os may be either retained in the mantle wedge or even returned to the deep mantle in the subducting slab.