OX-40⁺ sorted cells are alloreactive. OX40 sorted cells from liver were cultured in low-dose IL-2 and allowed to lose expression of the OX-40 antigen. The cells (day 12 postsort) were then cultured at 2 × 10⁴ cells/well in the presence of 10⁵ stimulator cells from the rats strains listed. After 2 days, ³H-thymidine was added and the cultures were incubated for an additional 18 hours. The cells were harvested and ³H incorporation was measured by liquid scintillation. The data are presented as the mean ± standard deviation of triplicate samples.