Abstract: The present invention is related to methods of assaying potency of a vaccine composition, wherein the potency is a pre-defined minimum level of potential biological activity for the vaccine composition. The method includes providing a vaccine composition and delivering same to an antigen presenting cell, wherein the vaccine composition is processed into peptides and the peptides are presented by MHC complexes on the cell surface. An agent, such as a T cell receptor mimic, that is reactive against a specific peptide/MHC complex is provided and reacted with the vaccine-treated antigen presenting cell, whereby the agent binds to the cell surface of the vaccine-treated antigen presenting cell if the specific peptide/MHC complex recognized by the agent is present on the cell surface. A density of the specific peptide/MHC complex on the surface of the vaccine-treated antigen presenting cell is measured by agent binding.