The ETS1 and ETS2 gene expression has been studied in purified human T-cells activated by either cross-linking of the T-cell receptor-CD3 complex on their cell surface or by direct stimulation with phorbol esters and ionomycin. The quiescent T-cells express high levels of ETS1 and undetectable levels of ETS2 mRNA and proteins. Upon T-cell activation, ETS2 gene products are induced while ETS1 mRNA and proteins decrease to very low levels. Late after stimulation, ETS1 mRNA is reinduced and maintained at a high level, while ETS2 gene expression decreases to very low levels. These results suggest that in human T-cells ETS2 gene products are associated with cellular activation and proliferation, while ETS1 gene products are preferentially expressed in a quiescent state. The reciprocal regulation of ETS1 and ETS2 genes is observed only in T-cells, but not in B- and non-lymphoid cells. Experiments are in progress to deprive ETS1 and ETS2 proteins to understand the specific role of c-ets gene products in thymocyte and T-cell differentiation, maturation, activation and proliferation. The pan-ets monoclonal antibody detects c-ets-1, c-ets-2, c-erg proteins in radioimmunoprecipitation assays, while c-ets-1 and additional 55 kDa proteins are detected by Western blot analysis. Multiple murine ets-1 (58,54,50 and 40 kDa) and ets-2 (56 kDa) have been identified as ets-1 and ets-2 gene-encoded products. Only 58, 54 kDa ets-1 and ets-2 nuclear proteins are hyperphosphorylated by Ca2+ mediated events. The origin of multiple ets-1 proteins, their post-translational modification and their functional significance are under investigation. The ERG genes are expressed at very low levels and their expression is restricted to a few cell types. They are expressed at high levels in immature rather than in mature myeloid cells. Different regions of ERG cDNA have been expressed in E. coli and purified proteins are being used to generate both polyclonal and monoclonal antibodies.