Rb, Sr and radiogenic 87 SR concentrations, and radioactivity "ages" have been determined by stable isotope dilution for lepidolite samples from 8 pegmatite occurrences. Five of the samples analyzed comprise an "intercomparison suite" ranging in age from 100 X 10 6 to 2600 X 10 6 years. Portions of these samples were sent by us to several other laboratories for analysis, as a test of the interlaboratory reproducibility of stable isotope dilution analysis. In this paper analytical data for this suite from 4 laboratories are compared. Interlaboratory reproducibility and intralaboratory reproducibility are essentially identical for the age determinations; it is apparent that age ratios for lepidolite can be measured routinely with analytical accuracy better than + or - 4% (standard deviation). Rb was determined with a precision of 1 to 2%, and Sr with a precision of 1 to 3% within individual laboratories; difference between results obtained at the several laboratories may be due in part to small differences in Rb and Sr standard solution calibrations and in part to sample inhomogeneities; the good interlaboratory agreement in age determinations favors the latter interpretation. A comparison of stable isotope dilution, optical spectrographic, and neutron activation Rb/Sr data for sets of independently collected and analyzed samples from 6 localities is also given. For each pegmatite, there is good agreement (a range of a few percent) in the ages determined by isotope dilution and neutron activation, although Rb concentrations have ranges of up to 30%. The study gives insight into the sources of error in the optical spectrographic technique and provides standards which may make possible the improvement of this method of analysis.