Two pairs of enantiomeric alkaloid diners, (+/-)-macleayins A (1) and B (2), representing a novel dimerization pattern of two different types of alkaloids via a C-C a-bond, were isolated from the aerial parts of Macleaya cordata. The enantiomeric separation was achieved;by chiral chromatography. Their structures and stereochemistry were determined by the analysis of extensive spectroscopic data, electronic circular dichroism calculation, and single-crystal Xray diffraction data. (-)-Macleayin A exhibits modest cytotoxic activity against HL-60 cell line with the IC50 value of 3.5I mu M.