We describe a highly sensitive, non-radioactive assay for T cell activation, based on the rapid induction of class II MHC expression by constitutively negative bovine endothelial cells, when cultured in the presence of supernatants derived from activated bovine T cells. We demonstrate the effectiveness of this assay in detecting rBoIFNy and activation of immune CD4 + and CD8 + T cell lines and clones in response to specific antigen and transfected COS-7 cells, respectively. We also demonstrate its utility in identifying purified pathogen fractions that activate immune CD4 + T cell clones.