Introduction An investigation was conducted to ascertain the potential carcinogenicity of several solvent refined paraffinic and naphthenic base stocks used in lubricating oils@ and of a composite motor oil before and after use. 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 lubricants (motor oils). 2. To determine the carcinogenic potential of paraffinic and naphthenic base stocks of different viscosities. 3. To provide limited information about other systemic toxic effects of skin exposure to each material. The assessments were made using data derived from the classical mouse skin painting bioassay. The following observations are included in this report: 1. The health of the mice throughout their lifetime. 2. The neoplastic and non-neoplastic skin lesions induced by each material tested. 3. The incidence@ rate@ and type of neoplasms. 4. Pathologic lesions in the internal organs.