10-Chloro-5-(2-dimethylaminoethyl)-7H-indolo [2,3-C]-quinolin-6(5H)-one hydrochloride (CIQ) was shown to exert significant antitumor activity against the Ehrlich carcinoma and sarcoma 180 transplantable tumors in mice by the intraperitoneal (ip) or oral (po) routes and when incorporated into diet. A solid tumor induced in BALB/c mice by the subcutaneous (sc) implantation of nonproducer murine sarcoma virus-transformed BALB/3T3 cells was also inhibited by CIQ after ip or po treatment but there was no effect against leukemia L1210 ascites or a transplantable murine renal adenocarcinoma. When tested in rats, CIQ significantly reduced the growth of Flexner-Jobling carcinoma, Murphy-Sturm lymphosarcoma and Walker 256 carcinosarcoma when administered by the ip or po routes. Pretreatment, but not posttreatment, with CIQ slightly inhibited the humoral antibody response of mice to sheep red blood cells. CIQ therefore differs from immunosuppressive agents such as imuran, methotrexate, cytosine arabinoside, or 6-mercaptopurine which affect the antibody response of mice to sheep erythrocytes when administered after immunization.