Chemical investigation of the seeds of Adonis aestivalis has led to the isolation of a new cardenolide (3β,5α,14β,17β-tetrahydroxycard-20,22-enolide) (1), two new glycosides (2, 3) of 1, and a new strophanthidin hexaglycoside (4), together with a known compound, strophanthidin 3-O-β-D-glucopyranoside (5). The structures of 1-4 were determined by 1D- and 2D-NMR spectroscopic analysis and the results of hydrolytic cleavage. The isolated compounds (1-5) were examined for their cytotoxic activity against neoplastic HSC-2, HSC-3, HSC-4, and HL-60 cells, as well as HGF, HPLF, and HPC normal cell lines. Compounds 2, 4, and 5 were found to display selective cytotoxicity toward malignant tumor cell lines. Although the morphological observations of HL-60 and HSC-2 cell deaths by 2, 4, and 5 revealed changes characteristic of apoptosis, neither DNA degradation nor activation of caspase-3 was observed. Our findings demonstrated that 2, 4, and 5 may trigger caspase-3-independent apoptotic cell death in HL-60 and HSC-2 cells.