Two new nor-pseudoguaianolides, microhelenin-E (1) and -F (2), were isolated from Texas Helenium microcephalum and their structures elucidated on the basis of physicochemical data and spectral evidence. Microhelenin-E demonstrated significant in vitro and in vivo cytotoxic and antileukemic activities against KB tissue cell culture (ED50 = 1.38 microgram/ml) P-388 lymphocytic leukemia growth in BDF1 male mice (T/C-166% at 8 mg/kg/day), respectively.