The G7 antibody reacts with the Thy-1.1 and Thy-1.2 alloantigens (CD90.1 and CD90.2, respectively) on thymocytes, peripheral T lymphocytes, some intraepithelial T lymphocytes (IEL, DEC), and neurons, but not B lymphocytes, of all mouse strains. CD90 is a GPI-anchored membrane glycoprotein of the Ig superfamily which is involved in signal transduction. In addition, there is evidence that CD90 mediates adhesion of thymocytes to thymic stroma. Soluble G7 mAb stimulates murine T-cell proliferation and IL-2 secretion, via signalling through the TCR/CD3 complex. mAb G7 has been reported to promote apoptosis of thymocytes and CTL clones, as determined by morphological and biochemical criteria. The Thy-1-triggered thymocyte death was found to be developmentally regulated and targeted mainly within the CD4+CD8+CD3- and CD4+CD8+CD3lo cultured immature thymocytes. G7 antibody does not react with Thy-1 (CD90) on rat thymocytes.