In multi-cellular organisms, failure to properly regulate cell-cycle progression can result in inappropriate cell death or uncontrolled cell division leading to tumor formation. To guard against such events, conserved regulatory mechanisms called "checkpoints" block progression into mitosis in response to DNA damage and incomplete replication, as well as in response to other signals. Checkpoint mutants in organisms as diverse as yeast and humans are sensitive to various chemical agents that inhibit DNA replication or cause DNA damage. This phenomenon is the primary rationale for chemotherapy, which uses drugs that preferentially target tumor cells with compromised checkpoints. In this study, we demonstrate the use of Drosophila checkpoint mutants as a system for assaying the effects of various DNA-damaging and anti-cancer agents in a developing multicellular organism. Dwee1, grp and mei-41 are genes that encode kinases that function in the DNA replication checkpoint. We tested zygotic mutants of each gene for sensitivity to the DNA replication inhibitor hydroxyurea (HU), methyl methanosulfonate (MMS), ara-C, cisplatin, and the oxygen radical generating compound paraquat. The mutants show distinct differences in their sensitivity to each of the drugs tested, suggesting an underlying complexity in the responses of individual checkpoint genes to genotoxic stress.