An investigation was conducted to ascertain the potential carcinogenicity of several base stocks, and oils before and after their use as lubricants. The samples were submitted by the American Petroleum Institute. The objectives of this experimental investigation were: (1) to determine the carcinogenic potential of 'New and Used Composite Motor Oils' (referred to as 'Used and New Lubricants'); (2) to determine the carcinogenic potentials of various base stocks of different viscosities; (3) to provide information about other systemic toxic effects that each material might induce. The assessments were made using data derived from the classical mouse skin painting bioassay. The following observations will be included in this report: (1) observations on the health of the mice during their lifetime; (2) the neoplastic and non-neoplastic skin lesions induced by each material; (3) the incidence, rats, and type of neoplasms; (4) significant pathologic lesions in the internal organs.