The origin and fate of small lymphoid cells in the dog bone marrow were studied autoradiographically by observing the effect of clamping of the femoral artery during in vivo availability of H3-thymidine. Heavily labeled small lymphoid cells appeared in the bone marrow of the clamped leg 3 hours after injection of the tracer and increased in number up to 6 days. The labeling indices of these cells, however, were significantly lower than those of control marrow. A possible interpretation is that dog bone marrow contains two populations of small lymphoid cells, one migrating into the marrow via the blood stream, the other originating from local precursor cells within the marrow. There was no evidence for a transformation of migrated small lymphoid cells into erythroblasts during the first 48 hours after injection of H3-thymidine.

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