Groups of male DBA/2 mice were irradiated with partial abdominal exposures of X-radiation ranging from 100-1600 rad. Concomitant with radiation exposure and at 1 or 4 h prior to, and at 1, 6, 24 or 48 h after irradiation, various chemotherapeutic agents were administered, i.e., methotrexate, cytoxan, adriamycin and BCNU [11,3-bis(2-chloroethyl-1-nitrosourea]. Excessive gastrointestinal toxicity may result if aggressive chemotherapy is closely spaced with radiation exposure for the treatment of abdominal neoplasms. Adjustment of dose and time patterns based on the proliferative responses of the mucosa may circumvent such toxicity to a large extent.