The regeneration of Kl from K2 neutral mesons and the mass difference between the two mesons were studied by passing a K/sup 0/ beam through 1.5- and 6- in. iron plates. About 200,000 pictures were made of the events, and they were analyzed for two-prong events where the primary momentum was equal to the beam momentum. The Q( pi , pi ) distribution of these selected events showed a peak around the expected value of 220 Mev, thus proving the regeneration of K1. The angular distribution of K1 decay events is plotted; the diffraction was computed from an optical model method and subtracted from the distribution to give the transmission peak, which was found to be confined to angles smaller than 2.5 deg (angle between primary K2 beam and regenerated K1). The mass difference is calculated to be zero for the 1.5-in. plate and 0.85 for the combination of the 1.5- and 6-in. plates. (D.L.C.)