To examine mechanisms of mobilization and homing of hematopoietic progenitor cells, coexpression of CD34 and the adhesion molecules L-selectin (CD62L), VLA-4 (alpha 4 beta 1-integrin, CD49d/CD29), and LFA-1 (alpha L beta 2-integrin, CD11a/CD18) was evaluated. Samples from leukapheresis (LP) products and bone marrow (BM) were obtained on the same day from patients who received granulocyte colony-stimulating factor (G-CSF) after cytotoxic chemotherapy. The proportion of CD34+ cells expressing L-selectin tended to be greater in LP products compared with BM. In samples from both sources, the mean fluorescence intensity of CD34 was significantly greater on CD34+/L-selectin-positive cells compared with the CD34+/L-selectin-negative cell subset. Three-color immunofluorescence showed that early CD34+/HLA-DRdim or CD34+/HLA-DR-progenitor cells were strongly positive for L-selectin, whereas L-selectin-negative cells were only found in the CD34+ HLA-DRbright subset. The mean fluoresence intensity of VLA-4 and LFA-1 was significantly greater on CD34+ cells from BM compared with LP products. Moreover, a distinct population of CD34dim/VLA-4bright and CD34dim/LFA-1bright cells was found only in samples from BM. This subset may be enriched for myeloid progenitor cells, since the cloning efficiency of CD34+ cells for CFU-GM was significantly greater in BM samples than in LP products. Binding of CD34+ cells to endothelial cells was partially inhibited by a blocking antibody to beta 2-integrin. In conclusion, L-selectin is expressed in significant amounts on more primitive CD34+ cells which circulate in considerable numbers in the peripheral blood. This suggests that L-selectin plays a role in redistribution and homing of hematopoietic progenitor cells to the bone marrow following cytotoxic damage. Conversely, strong expression of VLA-4 and LFA-1 was mainly found on lineage-committed progenitor cells of the bone marrow.