1. A range of commercial distillates of a topped crude Borneo, mineral oil were tested for carcinogenic activity and it was found that the peak was around a viscosity at 25° C. of 200 to 250 Centipoises.

2. The whole range of oils was biologically active. This activity had been foreseen from an examination of the physical characteristics, alone sufficient to condemn the utilization of any single representative as a lubricant when likely to come into contact with the user.

3. A greater concentration of the carcinogenic material was obtained by molecular distillation of oils than by ordinary distillation at from 3 to 10 mm.

4. The middle fractions of a Borneo crude oil were mostly responsible for pathological changes in the organs—fatty infiltration of the liver and hyaline degeneration of the spleen. These results again confirm previous findings.

5. Lung tumours were, according to our experience with mineral oils more prevalent than is usual when utilizing this type of agent for testing carcinogenicity for the skin. Their incidence was somewhat higher in the more viscous fractions but was not apparently related to their carcinogenicity for the skin.

In conclusion we should like to thank Dr C. C. Twort, former Director of this Department and now of the Portslade Laboratories, Sussex, for his helpful suggestions and advice in the compilation of this paper, and wish also to acknowledge their indebtedness to Dr J. R. Bowman of the Mellon Institute of Industrial Research, University of Pittsburgh, for a micro method of viscosity determination.