As part of an international measurement intercomparison of instruments used to measure astmospheric 222Rn, four participating laboratories made nearly simultaneous measurements of 222Rn activity concentration in commonly sampled, ambient air over approximately a 2-week period, and three of these four laboratories participated in the measurement comparison of 14 introduced samples with known, but undisclosed (''blind'') 222Rn activity concentrations that could be related to U.S. national standards. The standardized sample additions were obtained with a calibrated 226Ra source and a specially designed manifold used to obtain well-known dilution factors from simultaneous flow rate measurements. The exercise was conducted in Bermuda in October 1991. The 222Rn activity concentrations in ambient Bermudian air over the course of the intercomparison ranged from a few hundredths to about 2 Bq m-3, while the standardized sample additions covered a range from approximately 2.5 to 35 Bq m-3. The overall uncertainty in the latter concentrations was in the general range of 10% at a 3 standard deviation uncertainty interval. The results of the intercomparison indicated that two of the laboratories were within very good agreement with the standard additions and almost within expected statistical variations. These same two laboratories, however, at lower ambient concentrations, exhibited a systematic difference with an averaged offset of roughly 0.3 Bq m-3. The third laboratory participating in the measurement of standardized sample additions was systematically low by about 65--70%, which was also confirmed in their ambient air concentration measurements. The fourth laboratory, participating in only the ambient measurement part of the intercomparison, was also systematically low by at least 40% with respect to the first two aforementioned laboratories. ¿ American Geophysical Union 1995