Representative fields of cells showing unlabeled and labeled nuclei at 0 hours (A) and 30 hours (B) after cytokine stimulation of Lin⁻ Rho^low/Ho^low cells. Experimental procedure for the sorting of the stem cells, their labeling with ³H-thymidine, autoradiography, and Giemsa staining are as described in the Materials and Methods. Cells with labeled nuclei are indicated by arrows.