A method has been developed for the enrichment of the hematopoietic precursors of thymocytes from spleen and bone marrow cells. Up to 40-fold enrichments were obtained resulting in preparations in which as few as 10(5) cells produced prompt repopulation of the thymus of an irradiated mouse. Precursor cells from bone marrow appear to contain the enzyme terminal deoxyribonucleotidyl transferase (Tdt), an agent suggested as a potential somatic mutator. This enzyme (Tdt) was not detectable in any spleen cell preparation examined, including one in which a 40-fold enrichment of thymocyte precursors had been produced. This is the first difference reported between the splenic and bone marrow precursors of thymocytes.