Lead was extracted on a bromide-form anion-exchange column and loaded onto a single rhenium filament with silica gel and 0.1N H3PO4. U and Th were extracted on a nitrate-form anion-exchange column and loaded onto a triple rhenium filament assembly with 0.5N HNO3. Isotope ratios were measured on a NBS 30.5-cm mass spectromerer having digital control and data acquisition. Measured ratios were corrected for 0.10/a.m.u. discrimination. Lead laboratory blank ws <2 nanograms throughout the course of the study and was composed of 204Pb:206Pb:207Pb:208Pb = 1:18.7:15.6:38.7.