AbstractCytotoxicity of organotin compounds is assessed and their effectiveness against leukemia is discussed. The functional groups attached to the tin atom in organotin compounds control the cytotoxicity of the compound towards the thymus gland. The four organotin moieties which have the greatest toxic effect upon the thymus gland are the tri‐n‐butyltin, di‐n‐butyltin, tri‐n‐propyltin and di‐n‐octyltin groups. Compounds containing these groups also exhibit the poorest test‐control ratio (T/C) values when tested as anti‐cancer agents against leukemic mice