Ten-week-old C57B1/6J mice were given an intraperitoneal injection of 100 microCi radiostrontium (89Sr). One and two weeks following injection, the cellular composition of marrow and spleen was examined for cellularity, types of lymphoid cells, and incidences of small lymphocytes bearing sIgM, FcR, and CR. Strontium-89 treatment yielded a significant cell depletion in marrow, whereas spleen cellularity initially increased before returning to normal level in the second week. The increase in spleen cellularity was showed by pulse DNA labeling to be due to local production and not to influx from elsewhere. The percentage of medium and large lymphocytes, precursors of small lymphocytes, increased from 4% in the control spleen to 22% in the 89Sr-treated spleen. The incidences of small lymphocytes bearing sIgM, FcR, and CR were lower than normal levels with the exception of a substantial increase of FcR+ small lymphocytes in the spleen 2 weeks following 89Sr injection. Double-labeling studies, combining rosetting for surface receptors and DNA labeling for cell age, showed that small lymphocytes bearing sIgM, FcR, and CR form part of the major population of indigenous rapidly renewed cells in 89Sr-treated spleen.