On June 2, 1952, 15:52 hours, a large reactivity change was made manually in a ZPR-1 assembly causing a power excursion of about one kwh, which resulted in damage to the reactor core components and radiation exposure of some of the operating personnel to perhaps several rep. None of the contributing causes is reviewed here nor are the measures which were taken to reduce the probability of a recurrence discussed since these are considered administrative matters beyond the scope of this technical report. It is intended only to describe the incident, estimate the exposures and present available clinical data.