[en] One group of rats was injected intraperitoneally with ³HCdR and a second group was injected with ³HTdR. A third group received sheep red blood cells mixed with Freund's adjuvant in order to stimulate growth of germinal centers in the lymphoid tissues. Ten days later the third group was given ³HCdR or ³HTdR. The animals were killed at intervals up to 72 h after injection and the thymus, spleen, lymph nodes, and Peyer's patches of the intestine were prepared for autoradiography. The germinal center cells and thymus lymphocytes labeled much more intensely with ³HCdR than with ³HTdR. (U.S.)