Two series of compounds related to tepa and hempa were tested on male house flies, Musca domestica L. The 1st series consisted of bis (1-aziridinyl) alkylaminophosphine oxides, the 2nd series of tepa-hempa intermediates. The sterilizing effect and toxicity of the compounds in both series were determined. In the 1st series, the sterilizing effect decreased with increasing length of the carbon chain in the alkyl group, except for the isopropyl compound which was a more effective chemosterilant than the ethyl compound. In the 2nd series, the progressive replacement of the aziridinyl groups with dimethylamino groups led to a steady decrease in the sterilizing effect. The toxicity of compounds in either series showed no definite trend and was entirely unrelated to sterilizing Effect. A safety factor was defined and calculated for each compound.