The biodistribution of lymphocytes labeled with 109Pd was investigated in Lewis rats to determine if they might be useful for selective lymphoid ablation. 109Pd-labeled lymphocytes demonstrated significant lymphoid localization. However, there was a fall in the accumulation of radiolabeled lymphocytes in lymphoid tissue when the 108Pd carrier dose or the 109Pd radioactive dose incorporated per 10(8) lymphocytes was increased from 0.12 mg to 0.20 mg and from 21.3 microCi to 54.6 microCi, respectively (P less than 0.001). 109Pd-labeled syngeneic and allogeneic lymphocytes demonstrated similar tissue distribution patterns. These results raise the possibility of using 109Pd-labeled lymphocytes for selective lymphoid ablation, but emphasize the need for using high specific activity 109Pd and large amounts of lymphocytes for labeling. This will minimize cell damage and allow maximum therapeutic results to be obtained. The use of large numbers of cells might best be accomplished by using donor lymphocytes.