A method of identifying a target antigen of T cells comprising (a) contacting (aa) cells expressing (i) a functional T cell receptor complex comprising predefined matching T cell receptor α and β chains, wherein the functional T cell receptor complex is a complex capable of eliciting activation of the T cell in which the complex is expressed; and (ii) a read-out system for T cell activation; with (ab) antigen-presenting cells carrying (iii) peptide libraries encoded by randomised nucleic acid sequences; and (iv) MHC molecules recognised by the T cell receptor of (i); (b) assessing T cell activation using said read-out system; characterised by (c) picking single activating antigen-presenting cells that are in contact with the cells in which the read-out system indicates T cell activation and isolating the nucleic acid molecule encoding the peptide(s) of the peptide library present in said antigen-presenting cells; (d) identifying the target antigen or the nucleic acid molecule encoding said target antigen. (a) contacting (aa) cells expressing (i) a functional T cell receptor complex comprising predefined matching T cell receptor α and β chains, wherein the functional T cell receptor complex is a complex capable of eliciting activation of the T cell in which the complex is expressed; and (ii) a read-out system for T cell activation; with (ab) antigen-presenting cells carrying (iii) peptide libraries encoded by randomised nucleic acid sequences; and (iv) MHC molecules recognised by the T cell receptor of (i); (b) assessing T cell activation using said read-out system; characterised by (c) picking single activating antigen-presenting cells that are in contact with the cells in which the read-out system indicates T cell activation and isolating the nucleic acid molecule encoding the peptide(s) of the peptide library present in said antigen-presenting cells; (d) identifying the target antigen or the nucleic acid molecule encoding said target antigen. (aa) cells expressing (i) a functional T cell receptor complex comprising predefined matching T cell receptor α and β chains, wherein the functional T cell receptor complex is a complex capable of eliciting activation of the T cell in which the complex is expressed; and (ii) a read-out system for T cell activation; with (i) a functional T cell receptor complex comprising predefined matching T cell receptor α and β chains, wherein the functional T cell receptor complex is a complex capable of eliciting activation of the T cell in which the complex is expressed; and (ii) a read-out system for T cell activation; with (ab) antigen-presenting cells carrying (iii) peptide libraries encoded by randomised nucleic acid sequences; and (iv) MHC molecules recognised by the T cell receptor of (i); (iii) peptide libraries encoded by randomised nucleic acid sequences; and (iv) MHC molecules recognised by the T cell receptor of (i); (b) assessing T cell activation using said read-out system; characterised by (c) picking single activating antigen-presenting cells that are in contact with the cells in which the read-out system indicates T cell activation and isolating the nucleic acid molecule encoding the peptide(s) of the peptide library present in said antigen-presenting cells; (d) identifying the target antigen or the nucleic acid molecule encoding said target antigen.