Measured by its ability to co-stimulate IL-4 secretion by D10.G4.1 mouse helper T cells in the presence of anti-CD3. The ED50 for this effect is 0.3-1.5 μg/mL. Measured by its ability to stimulate human T cell proliferation in the presence of anti-CD3. Wang, S. et al. (2000) Blood 96:2808. The ED50 for this effect is 0.3-1.2 μg/mL.