Chimeric TCRs with an Ab-derived binding domain confer predefined specificity and MHC-independent target binding to T cells for use in adoptive immunotherapy. We investigated the impact of receptor binding affinity on the activation of grafted T cells. A series of anti-ErbB2 single-chain fragment binding domains with a K d ranging from 3.2× 10− 7 to 1.5× 10− 11 M was linked to CD3ζ-derived immunoreceptors and expressed in human PBL. Solid phase bound ErbB2 protein triggered activation of receptor-grafted T cells in a dose-dependent manner. The activation threshold inversely correlated with the affinity of the receptor binding domain. The maximum level of cellular activation, however, was the same and independent of the binding affinity. Upon binding to ErbB2+ cells, T cells grafted with immunoreceptors carrying a single-chain fragment of K d< 10− 8 M were activated in a similar fashion against cells with different amounts of ErbB2 on the surface. T cells with a low affinity receptor (K d> 10− 8 M), however, were activated exclusively by cells with high amounts of ErbB2. In conclusion, recombinant immunoreceptors of higher affinity do not necessarily induce a more potent activation of T cells than low affinity immunoreceptors, but the higher affinity immunoreceptors exhibit less discrimination between target cells with high or low Ag expression levels.