Human CD34⁺ bone marrow cell-conditioned medium stimulates elevated IL-6 synthesis. Bone marrow CD34⁺ bone cell-conditioned medium (CM) was prepared as detailed in the Materials and Methods over a 72-hour (0, 24, 48, and 72 hours) period. CM (80% vol/vol) was assayed for IL-6 stimulatory activity on confluent human osteoblast monolayers (HOB) grown in 96-well tissue culture plates at 72 hours by ELISA. Data from a representative of three experiments are presented as the mean ± standard deviation for triplicate determinations. IL-6 levels of CM before HOB exposure were ≤6 pg/mL (CD34 72 H CM). Basal production of IL-6 by HOB cells (HOB + 0 H CM). ⁺Significant difference from CD34⁺ conditioned medium alone. ^*Significant difference from HOB basal levels.