A method has been devised for simultaneously determining the concentrations of La, Ce, and Pr in carbon steels. The samples are dissolved and their solutions sent through small amounts of ion exchange resin. The dried resin is pelletized and the L alpha X- ray fluorescence intensities of the rare earths are obtained. The intensities are compared to a curve obtained from previously prepared elemental standards. Precision of the standard curves is 8-14 micron g, and detection limits are 0.002-0.004%, based on a 2- gram steel sample.