Chemical determination of the distribution of nitrogen in low-alloy, pressure-vessel steels of known response to neutron irradiation indicates a relation between the amount of uncombined nitrogen and the degree of radiation sensitivity. The sensitive steels that were tested were of varying microstructure but were distinguished by having almost all of the nitrogen combined as aluminum nitride. Four out of five of the insensitive steels contained at least 38 ppm uncombined nitrogen. For unknown reasons, the fifth insensitive steel did not fit the pattern established by all the others.