ADDRESSING a recent meeting of the Manchester University Branch of the Association of Scientific Workers, Mr. R. Lyth described how methods of preventing certain types of occupational cancer have been discovered. External cancers which are prevalent among Scottish shale oil workers have also increased among cotton spinners, since the introduction of mineral lubricating oils. In the Cancer Research Department of the University of Manchester, cancers have been produced in mice by repeatedly painting their skins with mineral oils. Shale oil was found to be much the most active carcinogenically, while Russian oil was one of the least active oils. The refractivity (µ- 1)/ D where µ is the refractive index and D the density, could be roughly correlated with the carcinogenic properties, and used as an index of them. In the laboratory, various methods of reducing the activity of oils were found, but the cotton industry has found it more practicable to avoid the use of the more dangerous oils such as shale oil. For the shale oil workers themselves, the regular use of certain ointments has proved beneficial. Efforts to isolate the carcinogenic constituents of mineral oils have not yet fully succeeded. The meeting learnt with regret that the Cancer Research Department of the University of Manchester is being closed.