Experiments into the carcinogenicity of 4,4'-methylenebis2- chloroaniline (101144) (MOCA) are reviewed. The carcinogenicity of MOCA is compared with that of 4,4'-diaminodiphenylmethane (101779) (DDM); MOCA is less toxic but more carcinogenic than DDM. Experiments with Wistar-rats fed a low protein diet containing MOCA demonstrate the relative nontoxicity of the compound over a 500 day period; the carcinogenicity is also demonstrated, with 23 of 25 males and 20 of 25 females dying with tumors. Other experiments are described in which 22 of 34 rats given subcutaneous injections of MOCA died of malignant tumors. Lung and liver tumors are cited in rats fed a normal diet containing 1000 parts per million MOCA for 18 months. The maximally tolerated dose in mice and rats is established at 2000 and 1000 milligrams per kilogram, respectively; mice and rats fed at these doses develop hepatomas, gliomas, adenocarcinomas and adenomata of the lungs, urinary bladder tumors, hemangiomas, hemangiosarcomas, and malignant lymphomas. Studies on workers are reported; negative findings were due to short durations of exposure and small sample populations. Two of six employees exposed to MOCA, toluene-diisocyanate (26471625), and other resins developed urinary symptoms, including hematuria for which exposure to MOCA was considered responsible. The author concludes that the results of experimental animal studies clearly demonstrate an active oncogenic role for MOCA. The chemical cannot be eliminated as a human carcinogen.