Ro 41-4439, a phenyl-pyridine-2-carboxylic acid derivative, was identified by a cell-based screening approach that exploits the differences between normal and cancer cells in their sensitivity to cytotoxic agents. This compound showed low micromolar antiproliferative activity and cytotoxicity against a broad panel of human cancer cell linesin vitro, and over 10-fold selectivity to cancer cells when tested in parallel with a panel of proliferating normal human cells. Cytotoxicity of Ro 41-4439 is due to arrest of cell cycle progression in mitosis followed by induction of apoptosis. Four-week treatment of nude mice bearing established mammary tumor xenografts (MDA-MB-435) with well-tolerated doses of the compound showed 73% inhibition of tumor growth. Limited exploration of structure–activity relationships involving side chain length, and aryl and pyridine rings allowed for the identification of more potent analogs.