Osmium isotopic analyses of picritic lavas from Hawaii show enrichments in the osmium-186/osmium-188 ratio (Os-186/Os-188) of 0.008 to 0.018%, relative to a chondritic upper mantle, that are positively correlated with enrichments in Os-187/Os-188 Of 5.4 to 9.0%. The most viable mechanism to produce these coupled Os-186 and Os-187 enrichments is by addition of 0.5 to 1 weight percent of outer core metal to a portion of the D " layer and subsequent upwelling of the mixture. These data suggest that some plumes originate at the core-mantle boundary and that Os isotopes may be used to distinguish plumes derived from shallow versus deep mantle sources.