A gas-source mass spectrometer has been constructed for the precise measurement of lead isotope ratios. Sixteen analyses on 4 different preparations of the same galena made over a period of 2 months gave 95% confidence limits (per analysis) of 206Pb/204Pb = 0.080%, 207Pb/206Pb = 0.042% and208Pb/206Pb = 0.046%. Eight samples from the U.S. mine in the Bingham district have a linear relationship over the 1% range of their 206Pb/204Pb ratios. The simplest model fitting these data suggests that the lead was separated from a primary system (??=8.98) 1630??150 m.y. ago and subsequently mixed with a radiogenic lead of similar age; biotite K-Ar dates for altered intrusive rocks associated with the ores provide an apparent age of mineralization and suggest that isotopic evolution of the lead was terminated about 36 m.y. ago. ?? 1967.