Changes in mechanical properties of A302-B reactor-grade, pressure-vessel steel due to various combinations of neutron irradiation, biaxial stress fields, and temperature were studied. Capsules containing tube, tension, and Charpy specimens were irradiated to doses from 1.6 x 10¹⁸ to 1.14 x 1020 nvt. Results of the Charpy tests are within the well-known scatter band of Steele, Naval Research Laboratory. Tube tests were performed at principal stress ratios of 1:1 and 2:1 at temperatures up to 149 C. The experimental results have been analyzed on the basis of the Hencky-Von Mises strength theory. A reduction of uniform elongation to less than 1 per cent was observed for tubes tested at 66 C, dose about 8 x 10¹⁸ nvt, and a stress ratio of 1:1. This should be compared with a uniform strain of 2 1/2 per cent for a tube tested in simple tension under the same conditions of temperature and dosage. The strain predicted from a simple instability criterion is about 2 per cent. The results indicate that the simple geometric methods generally used to predict instability under combined stresses greatly overestimate measured strains for this material. At doses of about 8 x 10¹⁹ nvt, yielding of tension bars was followed by large amounts of localized deformation indicating good ductility, but a condition of instability in a pressure vessel application.