In-situ deformation experiments have been performed in a 100-keV transmission electron microscope on aluminum which had been preirradiated with 600-MeV protons to a damage level of 1 to 5 dpa. The proton irradiation also produced helium and spallation product impurities. The intent was to determine which feature present in the irradiated microstructure produced the radiation strengthening observed earlier. The microstructure features resolved before deformation included small helium bubbles and dislocations. In-situ stressing produced dislocation motion. In observations performed during stress relaxation, local dislocation curvature indicated local strengthening. The strongest type of dislocation barrier was a presumed submicroscopic cluster defect; the helium filled bubbles were less effective obstacles to dislocation motion.