The southwestern Indian paintbrush, Castilleja linariaefolia, yielded extracts that displayed in vivo activity against murine P-388 (PS) lymphocytic leukemia. Separation guided by PS cell line inhibition led to isolation of cytotoxic compounds that were identified as the known glycosides acteoside [1] (ED50 2.6 micrograms/ml) and isoacteoside [2] (ED50 10 micrograms/ml). The identifications were established by spectral measurements and degradation studies. Mannitol was also found in this plant.