The direct assessment of T helper (T(H))-cell responses specific for antigens is essential to evaluate pathogenic and protective immunity. Presently, analysis and isolation of antigen-specific T(H) cells is restricted to cells that produce cytokines, or can be performed only with a rare selection of specific peptide major histocompatibility complex class II (MHC II) multimers. Here we report a new method that enables the assessment and isolation of T(H) cells specific for a defined antigen according to CD154 expression induced after stimulation in vitro. We show that antigen-induced CD154 expression is highly sensitive and specific for human and mouse antigen-specific T(H) cells. Moreover, the isolation of antigen-specific CD154(+) T(H) cells necessitates only surface staining with antibodies, thereby enabling the fast generation of antigen-specific T(H) cell lines. Our approach allows assessment of T(H) cells with a defined specificity for the combined quantitative and qualitative analysis of T(H)-cell immunity as well as for the isolation of specific T(H) cells for targeted cellular immunotherapies.