A series of artemisinin-related endoperoxides was tested for cytotoxicity to Ehrlich ascites tumor (EAT) cells using the microculture tetrazolium (MTT) assay. Artemisinin  had an IC50 value of 29.8 microM. Derivatives of dihydroartemisinin , being developed as antimalarial drugs (artemether , arteether , sodium artesunate , artelinic acid , and sodium artelinate ), exhibited a somewhat more potent cytotoxicity. Their IC50 values ranged from 12.2 to 19.9 microM. The presence of an exocyclic methylene fused to the lactone ring, as for artemisitene , led to higher cytotoxicity than 1. From the two epimeric 11-hydroxyartemisinin derivatives, the R form 12 showed a considerably higher cytotoxicity than the S form 13. Opening of the lactone ring of 1 dramatically reduced the cytotoxicity. The ether dimer 8 of 2 was the most potent cytotoxic agent, its IC50 being 1.4 microM. The variations in cytotoxicity between the structurally related compounds mostly correlated well with the theoretical capacity of radical formation and stabilization. In some cases lipophilicity or the presence of an electrophilic moiety seemed to have a determinant influence on cytotoxicity. The artemisinin-related endoperoxides showed cytotoxicity to EAT cells at higher concentrations than those needed for in vitro antimalarial activity, as reported in the literature.