Cell- to-cell contact is not required for elevated IL-6 synthesis. CD34⁺ bone marrow cells (CD34⁺; 1.0 × 10⁴) were seeded either directly onto confluent osteoblast monolayers (HOB; contact cultures), the top chamber of dual-chambered 24-well plates with confluent primary human osteoblasts in the bottom chamber (noncontact), or alone (CD34⁺) in Ham's F12/DMEM (1:1 vol/vol) containing 1% FBS. In some cases, 2% paraformaldehyde was used to mildly fix the osteoblasts before the initiation of coculture experiments (fixed HOB). At 96 hours, conditioned medium was collected and assayed for IL-6 by ELISA. Data from a representative of three experiments are presented as the mean ± standard deviation for triplicate determinations. ^*Significant difference from HOB. ⁺Significant difference from CD34⁺ or fixed HOB cultures to a statistical significance of P < .05.