CD3 is presented on all resting and activated human T-cells, on T-leukemia cells and a proportion of human thymocytes. CD3 plays an important role in the assembly and expression of the T-cell receptor complex. Furthermore it functions as a signal transductor. SPV-T3b reacts with human CD3 also designated as T3 with a molecular weight of 20-26 kDa. SPV-T3b recognizes also the T3 molecular complex.