A high frequency of fresh CD34⁺/CD86⁺ and CD34⁺/CD86⁺-derived cells cultured in TNF-α, GM-CSF, and SCF stimulate the proliferation of allogeneic T cells. Fresh (A) and TNF-α, GM-CSF, and SCF-cultured (B) CD34⁺/CD86⁺ and CD34⁺/CD86⁻ cells and were irradiated (3,000 cGy) and mixed with allogeneic, nylon wool-purified T cells at 10,000 responders/well. Stimulator cells were serially diluted and set up in 96-well V-bottomed plates in 24 replicates. Similar results were obtained in three experiments of identical design. For the experiment shown, the χ² and frequency (f) of stimulating cells in each population were as follows: fresh CD34⁺/CD86⁻ cells, χ² = 2.0, f = 0.6% (95% confidence interval [CI], 0.5% to 0.8%); fresh CD34⁺/CD86⁺ cells, χ²=4.7, f = 3.1% (95% CI, 2.4% to 3.8%); cultured CD34⁺/CD86⁻ cells, χ² = 1.33, f = 4.0% (95% CI, 3.0% to 5.0%); and cultured CD34⁺/CD86⁺ cells, χ² = 0.68, f = 25.0% (95% CI, 16.7% to 33.3%).