Gel filtration analysis of FR secretion by KB and CD34⁺ cells. KB cells (▿) and CD34⁺ cells (▾, ○) were incubated for 2 hours (▿, ▾) or 20 hours (○) in FDMEM containing 50 nmol/L ³H-FA. After incubation, the growth medium was collected, and protein bound ³H-FA was separated from free ³H-FA by gel filtration chromatography on a Sephadex G-25 column. For comparison, fresh (unconditioned) FDMEM was treated with ³H-FA and evaluated similarly (•). Protein bound ³H-FA eluted at ≈3 mL, while free³H-FA eluted beyond 5 mL. Although considerable protein-bound ³H-FA was apparently generated during the 2-hour incubation of KB cells, no measurable increase in³H-FA binding capacity was detected in either the 2-hour or 20-hour supernatant of the CD34⁺ cells above the level present in the unconditioned FDMEM.