[en] In 1977 the U.S. Environmental Protection Agency distributed calibrated solutions of plutonium-239 to laboratories interested in participating in an intercomparison study of plutonium analysis. Participants were asked to perform a quantitative radioactivity analysis of the solution. The results reported by all the participating laboratories are given here. Eighty-eight percent of the reported activity values were within plus or minus 20 percent of the activity value certified by the Environmental Protection Agency