The remainder of the lanthanum-140 is not lost but stays with the barium-140.The quantitativeness of precipitation of barium nitrate in 80% nitric acid was determined by analyzing the filtrates for barium-140 by the method of differential decay (6). The average barium concentration in the 80% nitric acid filtrates was 0.01 mg. of barium per milliliter. Since the total volume was 5 ml. and about 250 mg. of barium carrier was used, the amount of barium-140 remaining in the filtrate as a radioactive impurity in the lanthanum-140 was found to be approximately 0.03%.The recommended procedure is particularly effective for preparing carrier-free lanthanum-140 because only strontium, barium, radium, and lead form insoluble nitrates in 80% nitric acid (21). Hence, the lanthanum-140 which grows in after the first milking need be separated only from these elements and their daughter activities. Radium and lead were both found to be absent, as the alpha activity in the lanthanum-140 mounts did not differ significantly from background. (All radioactive isotopes of lead and radium have alpha emitters in their decay chains.) The quantity of barium has been shown to be very low, and the same is probably true for strontium (21). However, if