The Bureau of Mines Helium Field Operations has developed a method for determining the helium-3 (3he) content of natural gas and mixtures containing 0.5 to 100 pct helium. The helium is preconcentrated by passing samples through anhydrone, a liquid- nitrogen-cooled charcoal trap, and a titanium absorption trap. The mass spectrometer used to analyze the samples is tuned to 3he at m/e = 3.016. Hydrogen deuteride (hd+) interference is minimized by the titanium absorption trap and high instrument resolution. Sample measurements are compared with identically made measurements of a gravimetrically prepared primary standard. The method has a limit of detection of 0.15 ppb with a relative deviation of 13 pct below the 1-ppb level. The relative deviation is less than 7 pct for gases containing over 100 ppb 3he. The helium isotope ratios of natural gases from the Bureau's Tuck-Trigg Dome Red Cave, the Bush Dome Red Cave, and the Bush Dome Brown Dolomite Formations are distinguishable from one another and from the isotope ratio of crude helium produced from the Panhandle-Hugoton gasfield.