Pyrimidinones are low-molecular-weight compounds which are inducers of interferon in several animal species. They have established antiviral, immunomodulatory, and antitumor effects. Four pyrimidinones as well as another potent interferon inducer, polyriboinosinic-polyribocytidylic acid, and beta-interferon were tested for effects on growth of the transplantable mouse bladder tumor (MBT-2). The pyrimidinones 2-amino-5-bromo-6-phenyl-4(3H)pyrimidinone (ABPP) and 2-amino-5-bromo-6-(3-fluorophenyl)-4(3H)pyrimidinone (ABMFPP) significantly inhibited MBT-2 growth in a dose-dependent manner and with equal potency when injected i.p. every 4 days starting 1 day after tumor cell inoculation. Administration of ABPP p.o. was as effective as i.p. injections. Direct intravesical application of ABPP to transplantable tumors growing in the bladder may be more effective in inhibiting MBT-2 growth than the same dose introduced p.o. Although ABPP (100 mg/kg) has an inhibitory effect comparable to 5000 units of beta-interferon, both pyrimidinones even at 500 mg/kg were less inhibitory of tumor growth than 10 mg of polyriboinosinic-polyribocytidylic acid per kg. The pyrimidinones 2-amino-5-bromo-6-(2,5-difluorophenyl)pyrimidine-4(3H)one (ABDFPP) and 2-amino-5-iodo-6-(2,3-difluorophenyl)pyrimidin-4(3H)one (AIDFPP) were also of comparable potency in inhibiting MBT-2 growth and were more effective on mg/kg basis than both ABPP and ABMFPP. Treatment with ABDFPP or AIDFPP also resulted in long-term cures of up to 40% of mice. In this respect these latter two compounds were superior to treatment with 10 mg of polyriboinosinic-polyribocytidylic acid per kg, a treatment which reduced tumor size but had no effects on tumor incidence. The data suggest that tumors of bladder origin may be particularly sensitive to treatment with pyrimidinones.