The present invention relates to zinc free insulin crystals having a diameter below 10 &mgr;m and to therapeutic powder formulations suitable for pulmonary administration containing such insulin crystals. The crystals of the present invention exhibit a better stability profile than powders of essentially the same composition prepared by spray drying, freeze-drying, vacuum drying and open drying. The therapeutic powder formulations elucidate better flowing properties than corresponding amorphous powder formulations.