The composition of garnet in each of 11 specimens from anorthosite–charnockite suites (Marcy Massif, Morin Complex, Lac-Croche Complex) has been determined by electron-probe microanalyzer. All analyses include Si, Al, Fe, Mn, Mg, and Ca. The range of FeO (total iron as Fe2+) is from 26.5 to 34 wt.%; MnO from 0.5 to 2, MgO from 0.5 to 5, and CaO from 7 to 9.5 wt.%. TiO2 is present in very small amounts at most (?). The major end member molecule in all specimens is almandine.Bias and precision of the analysis have been estimated: (1) by replicate determination of the composition of a garnet standard in each analytical run on each of the unknown species and (2) by duplicate determination of the garnet species in each rock specimen.In general, inclusion of a known species, similar in composition and physical state to the unknown species, in each analytical run on an unknown will provide estimates of bias applicable to individual analyses as well as an overall estimate of bias applicable to the mode and circumstances of the particular series of runs on the electron-probe microanalyzer. The reliability of the estimates of bias so obtained depends mainly on the compositional homogeneity of both known and unknown species.