The utility of 238Pu produced from 237Np irradiation depends on the amount of 236Pu present. Calculations of 236Pu produced by the (n,2n) reaction on 237Np indicate that light-water power reactors produce 238Pu containing 10 to 20 times as much 236Pu as that produced in the USAEC D2O-or graphite-moderated plutonium production reactors, and the liquid-metal fast breeders produce 300 to 400 times as much 236Pu as the production reactors.