AbstractA homemade rust‐proof cutting fluid (RPCF) used in China was tested for carcinogenicity by an in vivo chronic experiment and for mutagenicity by the AmesSalmonellamicrosomal assay. Undiluted and threefold water‐diluted fluid were given as drinking water to groups of young adult Wistar rats for 2 years. The treatment induced 11/40 malignant tumors with 9/40 acinar adenocarcinomas of the pancreas in the high‐dose group. Simultaneous administration of ascorbic acid dissolved in the undiluted fluid at 2 g acid per 1 g sodium nitrite resulted in 1/40 pancreatic carcinoma. The results of the Ames test showed that the technical RPCF was mutagenic to TA100 with or without metabolic activation. It was concluded that the homemade RPCF, which is comprised of sodium nitrite, triethanolamine, and polyethylene glycol, may form direct‐acting mutagen(s) upon storage and form, in vivo, e.g., nitrosamines that caused acinar pancreatic carcinoma in Wistar rats. Simultaneous administration of ascorbic acid is suggested for the protection of workers exposed to the rust‐proof cutt