The RUC and government scientists were alerted last night after a canister of radioactive gas went missing from a store at an industrial complex in Bangor, Co Down.
The gas used by 3M Industrial Tapes in the production of adhesive film tapes, was in a steel case within a wooden box, marked with warning signs.
An immediate investigation has been ordered by the Department of the Environment's Alkali and Radiochemical Inspectorate.
A spokesman last night said: "There is minimal radiation hazard to the general public but any person who is in close contact with the radioactive source for a long period might suffer localised burning and other detrimental effects."