This review of historical controls was motivated by an October 1983 report of Kettering Laboratory entitled 'The Carcinogenicity of New and Used Lubricants.' In that study, three control groups were run: two groups of shaved, untreated controls and one group of toluene-treated controls. No tumors developed in the untreated controls, and one in the toluene-treated group. In the unused composite motor oil group and in four of the five paraffinic oil groups tested in the study, two tumors each were diagnosed. The incidence in these groups was compared to concurrent controls and they were judged to be 'no more than slightly carcinogenic.' Because this statement implies some carcinogenic potency to the samples, API requested that Kettering reexamine the incidence of tumors in historical controls to determine whether the controls in this study were representative. The results of that examination show that the occurrence of two tesors in control groups is not unexpected, and, because of this, no haputation of carcinogenicity can be made with regard to these samples.