Flow cytometric analysis of PB obtained from a representative chimera 30 days after reconstitution with mobilized PB. B10 mice (H-2K^b) were lethally irradiated and transplanted with varying numbers of PBMC from growth factor–treated B10.BR donors (H-2K^k; 4 to 7 mice per group). PB from unmanipulated B10 and B10.BR mice served as controls. Lineage derivation of PBMC was analyzed based on forward and side scatter, and the percentage of cells residing in a lymphocyte (R1), monocyte (R2), or granulocyte gate (R3) was calculated. (A) The majority of PBMC in engrafted recipients were located in the granulocyte gate, (B and C) while most of PBMC from untreated controls resided in the lymphocyte gate. (D through F) In addition PB was stained with MoAb specific for recipient (H-2K^b) and donor (H-2K^k) MHC class I, and gated populations were analyzed by two-color flow cytometry. (D) Gated lymphocytes from engrafted recipients expressed exclusively donor MHC class I. Positive staining for donor but negative staining for recipient MHC class I was also observed when gated granulocytes and monocytes were analyzed (data not shown).