Collect 0 to 2 day old adult males and let them age a further 2 days at 25 C. Aliquot the flies into groups of 50 each. Place each group of 50 into a gelatin capsule (Lilly, No.000) which has been perforated with a 23 gauge needle. Put the capsules in a petri dish on ice. Make sure the flies get cool. Take care that the flies and capsule do not get wet. Place the dish in the X ray machine and give the flies a 4000 rad dose (115 kV; 5 mA). Alternatively, place males in empty vials and lay vials on their sides in x-ray machine before giving them a 4000 rad dose. Immediately transfer the flies to a vial with plenty of yeast and allow them to recover for a few hours. Let the males mate with an equal number of 3 day old virgin females. I like to put 30 of each sex in a bottle. Knock the flies into fresh bottles on successive days and on the fourth day discard the flies. Starting with 400 males, one can easily get ten to twenty thousand F1 flies to screen.

original text by Richard Carthew