A canister of radioactive material has been stolen from the back of a van, police have said.
The "small amount" of material, believed to be Iridium 192, is not considered dangerous so long as it remains contained in its heavy lead container.
Lancashire police said the yellow canister, which is marked with the radiation trefoil, was taken some time over the weekend from a Peugeot Panel Van which was parked outside an address on New Line in Bacup.
Chief Inspector Russ Procter said: "It is important that this material is located as soon as possible. I would ask the public, especially in the Rossendale area, to keep an eye out for this canister and if they locate it then to call the police who will come and deal with it.
"I would also appeal directly to the people who have stolen this. They may have no idea what it is that they have in their possession or they may have discarded it somewhere. If that is the case then I would ask them to contact us or call Crimestoppers anonymously."
Police have not said what the purpose of the material was.
The Health Protection Agency urged anyone coming into contact with the cylinder to leave it where it is and alert the police.
A spokesman added: "The radioactive material, believed to be Iridium 192, will not pose a risk to the public if it remains contained in its heavy lead container marked with the radiation trefoil.
"However, if the material is outside of its protection packaging, anyone who has come into prolonged physical contact, such as keeping it in a pocket, should seek medical assistance. If you see this package you should immediately inform the police, do not touch it and keep away."
Anyone with information should call Lancashire police on 999 or call Crimestoppers on 0800 555 111.