The plant chemistry and cytotoxic activity of Colchicum baytopiorum CD Brickell (Liliaceae/Colchicaceae), an endemic species growing in Turkey, has been studied for the first time. Nine known alkaloids were isolated and their structures were identified by spectral methods (UV, IR, (1)H-NMR, and ESI/MS), and the presence of three alkaloids, which could not be isolated from the plant, was also detected by LC/MS/MS spectrometry. Phenolic acids were elaborated using LC/MS and 11 phenolic acids were identified. The presence of two flavonoids appeared to be valuable for chemotaxonomic purposes. Guided by the brine shrimp lethality test (BSLT), methanol extracts were tested for cytotoxic activity by colorimetric MTT test on K562 and HL60 cells. Except the seed extract, all methanol extracts showed more cytotoxic activity on HL60 cells (IC(50) : 6.5-< 0.1 mu g/mL) than on K562 cells (IC(50) : > 500-44 mu g/mL).