The interactions of radiation and adriamycin (ADM), bleomycin (BLM), mitomycin C (MM-C), or cis-diamminedichloroplatinum II (cis-DDP) in mouse jejunal crypt cells were studied using the microcolony survival assay. ADM administered from 24 h before to 48 h after irradiation resulted in an almost constant enhancement of the radiation response, the dose effect factor (DEF) being 1.19. The effect of BLM was extremely dependent on the sequence and interval between drug administration and irradiation. The most pronounced effect was observed when BLM was given 2 h before irradiation (DEF = 2.40), at which interval the D0 surprisingly increased by a factor of 1.4. Administration of MM-C from 24 h before to 24 h after irradiation enhanced the radiation response. The effect peaked on administration 6 h before irradiation (DEF = 1.21) and diminished by application after irradiation. Cis-DDP enhanced the radiation response only when given before irradiation resulting in a DEF of 1.23 and a decreased D0.