Stromal production of IL-6 and G-CSF is stimulated by CD34⁺ hematopoietic progenitors. Irradiated marrow stromal layers were subcultured with LTBMC medium in 24-well plates as described. A total of 10,000 CD34⁺/HLA-DR⁺or CD34⁺/HLA-DR⁻ cells/well were seeded directly onto the stromal layers (stroma contact, SC) or in a transwell insert (stroma noncontact, SNC) on day 0. Cytokine levels were determined by ELISA in SNs obtained on days 2, 4, and 7 during the first week and on day 2 after a half medium replacement during weeks 3, 4, and 5. (A) Concentration of IL-6 in SN. Numbers within the figure indicate number of experiments. Comparison between stroma only and stroma plus progenitors (DR⁺ or DR⁻ cells in either contact or noncontact): *P < .05. (B) Concentration of G-CSF in SN. Numbers within the figure indicate number of experiments. Comparison between stroma only and stroma plus progenitors (DR⁺ or DR⁻ cells in either contact or noncontact): *P < .05. (C) Comparison between contact and noncontact cultures on G-CSF and IL-6 concentrations. N = 5.