Following the detonation of the experimental atom bomb at Alamogordo, New Mexico, on July 16, 1945, a radioactive contaminant was encountered in strawboard material used by the Eastman Kodak Company for packaging photographic sensitive films. This paper board was manufactured in a mill situated at Vincennes, Indiana, on the Wabash River. A run of strawboard, produced on August 6, 1945, showed this new and unusual type of radioactive contaminant. X-ray film packed with this board showed fogged spots after about two weeks' exposure. Measurements of the contaminated spots of strawboard showed no alpha-activity but fairly strong beta-activity. Absence of alpha-activity ruled out naturally radioactive materials. Measurements of the beta-activity showed a maximum energy of about 0.6 Mev. and a half-life of approximately 30 days. Radiochemical studies of the active ash from the strawboard indicated that the material was of the rare earth series. The energy value and half-life of the beta-radiation are compatible with the isotope, Ce 141. All studies point to the conclusion that the radioactive contaminant was an artificially radioactive material which found its way into the mill through the river water. The most likely explanation seems to be that it was a wind-borne fission product derived from the atom-bomb detonation in New Mexico on July 16, 1945.