Mice were whole-body-irradiated with 5 fractions of roentgen rays in 5 days, 5 fractions of cyclotron neutrons in 5 days, or with mixed neutron-photon fractionated radiation, in the sequence n-n-x-x-x or n-x-x-x-n. The LD50/4 day values were determined. Roentgen rays and neutrons interact in the additive manner in the mixed fractionation schemes: effective dose per fraction is as predicted from the roentgen ray-only and neutron-only experiments. This essentially agrees with HENDRY et coll. (1976). However, no trend was found towards a less-than -additive effect which was observed by those authors and has also been suggested in skin response to mixed schemes (NELSON et coll. 1975).