The extraction of alkaline earth cations by xylene solutions of dinonylnaphthalenesulfonic acid (HDNNS) from aqueous hydrochloric acid solutions containing various aqueous-soluble unsubstituted crown ether ligands (15-crown-6 (15C5), 18-crown-6 (18C6) and 21-crown-7 (21C7)) has been investigated. The extraction results indicate that the extraction of the larger alkaline earth cations by HDNNS is strongly synergized by the presence of the crown ethers in the aqueous phase. Promising results for intra-Group-IIa cation separations have been obtained with each of the three crown ethers investigated. Separation factors for the radium-calcium couple of at least 10(2) are obtained under a number of conditions, while the separation factors within the radium-barium-strontium series are much lower. Possible problems in developing an extraction chromatographic resin for radium separation and preconcentration based on the extraction data are discussed.