Methylene-bis-orthochloroaniline (MOCA) induced a wide spectrum of neoplasms in male rats fed either a protein-adequate (27 percent casein) or a protein-deficient (8 percent casein) diet. The concentrations of MOCA used were 125, 250, 500 and 1000 ppm. Increasing doses of MOCA in either diet resulted in decreased survival times. MOCA induced pulmonary adenomas, adenocarcinomas, mammary gland adenocarcinomas, Zymbal gland carcinomas, hepatocellular carcinomas, and hemangiosarcomas. In both diet groups the lungs were the most sensitive organs to the induction of neoplasms by MOCA. The incidence of primary pulmonary neoplasms in the lowest dose tested (125 ppm) was 6 percent (p less than or equal to 0.01), while in the highest dose (1000 ppm) it was 70 percent (p less than or equal to 0.01). The hepatocellular carcinoma incidence in rats fed a protein-deficient diet with 500 ppm MOCA was 18 percent, whereas in rats fed a protein-adequate diet with the same MOCA concentration this incidence was only 4 percent. The mean urinary concentration of MOCA in the group of rats fed the lowest dose (125 ppm-PD) was 0.63 ppm, a concentration comparable to that measured in the urine of workers exposed to MOCA.