IL-1β and TNF-α production by CD34⁺ cells is not primarily responsible for increased IL-6 synthesis by HOB cells. Bone marrow CD34⁺ cells (0 to 1.0 × 10⁴) were seeded directly onto confluent human osteoblast monolayers (HOB) in 96-well tissue culture plates containing 1% FBS. For these experiments, daily addition of murine neutralizing monoclonal IgG₁ antibodies to human IL-1β, TNF-α, isotype-matched control, or vehicle were added at concentrations approaching 10 times the anticipated neutralizing dose (ND₅₀ ) reported by the manufacturer (10 μg/mL). At 96 hours, conditioned medium was collected and assayed for IL-6 by ELISA. Data from a representative of two experiments are presented as the mean ± standard deviation for triplicate determinations.