Calcic plagioclases from basaltic-gabbroic rocks and anorthosites exhibit a strong fractionation of K and Rb. The trend of K Rb ratio with K content is similar to that shown by oceanic tholeiites. Such a K-Rb relationship is not shown by metamorphic plagioclase. High K Rb ratios in gabbroic or anorthositic rocks may not indicate a "mantle" origin but may only reflect partitioning of K and Rb between liquid and plagioclase. The K Rb ratios in stone meteorites appear to be largely controlled by their constituent plagioclase.