A method of assaying for peptide-specific T-cells comprises adding peptide to a fluid sample of fresh peripheral blood mononuclear cells, and detecting a cytokine such as interferon-y produced by T-cells that have been pre-sensitized to the peptide. The assay method is quick and cheap and is expected to be useful for the study of various disease states including Hepatitis B, Hepatitis C, tuberculosis, malaria, HIV and influenza.