We have studied the ability of bone marrow cell suspensions greatly differing in the relative proportion of day-7 and day-12 spleen colony-forming units (CFU-S) to rescue mice from radiation-inflicted death, and to repopulate the irradiated bone marrow and spleen with nucleated cells. Counterflow centrifugal elutriation in combination with removal of adherent cells and fluorescence-activated cell sorting on differences in wheat germ agglutinin (WGA)-fluorescein isothiocyanate (FITC) affinity and light scatter properties were used consecutively to enrich large numbers of hemopoietic stem cells from mouse bone marrow. Enrichments of 50-to 200-fold have been achieved for day-12 CFU-S and radioprotective ability (RPA), permitting 50% of lethally irradiated mice to survive over a period of 30 days with as few as 50-80 donor cells. The ratio of day-7 and day-12 CFU-S in the various suspensions could be significantly modulated on the basis of their WGA binding and perpendicular light scatter characteristics. This finding enabled us to investigate the properties of day-7 and day-12 CFU-S with respect to their RPA. We found a highly significant log/log relationship between enrichment factors for (1) RPA,(2) the number of day-12 CFU-S, and (3) spleen cellularity as measured on day 13. In addition, similar numbers of sorted and unfractionated day-12 CFU-S were required to obtain the same level of protection. Enrichment for RPA was significantly less related to either the number of day-7 CFU-S injected, or the bone marrow cellularity of the irradiated mice on day 13.