Adult male Fischer rats were exposed to a necrogenic dose (200 mg/kg) of diethylnitrosamine or to nonnecrogenic doses of N-methyl-N-nitrosourea, 1,2-dimethylhydrazine, or benzo(a)pyrene following partial hepatectomy or sham hepatectomy. This treatment by itself led to no hepatocellular carcinomas by 8-18 mo., except in animals given N-methyl-N-nitrosourea, which showed a 30% incidence by 12 mo. With each treatment regimen, exposure to dietary 2-acetylaminofluorene for 2 wk coupled with partial hepatectomy or the administration of a necrogenic dose of CCl4, was associated with an incidence of 68-94% of cancer at 8, 12, or 18 mo., depending upon the initiating carcinogen used. Appropriate controls showed either no hepatocellular carcinoma or a much lower incidence. The 2-wk exposure to dietary 2-acetylaminofluorene plus partial hepatectomy or the administration of CCl4 has a strong promoting effect on liver carcinogenesis with 4 different chemical carcinogens.