Immunofluorescent labeling of 16.5-dpc fetal liver cells with anti-runt box antiserum either alone (A and B) or in combination with MoAbs specific for granulocytes (anti-Gr-1; C and D) or erythroid cells (TER-119; E and F ). The anti-runt box reactivity appears as red-orange stain (R-phycoerythrin); reactivity with lineage-specific antibodies appears in green (FITC). (A) Anti-runt box staining. (B) Preimmune rabbit serum control. Competitions with GST or GST/runt (C through F ) were performed as described in the legend to Fig 2. In (C), the arrow indicates a large blast of granulocytic lineage (green staining) that was strongly labeled by the anti-runt box (red-orange); this last staining is prevented by preincubation with GST/runt. In (E), the double-labeled cells (arrowhead) show expression of runt-box polypeptides in the erythroid lineage, albeit at much lower levels than in myeloid blasts (long arrow). Even in these cells, specificity of the anti-runt box stain is shown by competition with GST/runt (F ).