Unirradiated, neutron-irradiated, and in situ helium ion-irradiated stainless steel specimens have been tension-tested in a high-voltage electron microscope at temperatures from 25 to 600°C. The relative ductility of these specimens was assessed as a function of temperature and correlated with transmission electron microscope observations of crack-tip angles, plastic zone widths, and crack propagation characteristics. In neutron-irradiated specimens tested at 25 to 400°C, the cracks propagated along channels and slipbands. The fracture surface of the 400°C test specimen showed coarse slip steps up to 5 μm wide. Coarse slip was sometimes followed by homogeneous slip on different slip systems. The presence of large voids or helium bubbles seems to prevent cracks from following channels and to cause them instead to follow the zigzag path typical in ductile specimens.