Although widely used, penicillins cause a range of adverse drug reactions ranging from cutaneous reactions to anaphylaxis in 1-10% of individuals treated. These reactions are generally classified as idiosyncratic and involve inappropriate activation of the immune system during drug administration. Depending on reaction phenotype, a range of immune cell types have been implicated, including both CD4 and CD8 T cells. In healthy individuals, CD8 and CD4 T cells are normally activated via the presentation of pathogen derived peptides on the surface of antigen presenting cells by MHC class I and II molecules respectively. In contrast, self-derived peptides presented by the MHC are usually ignored. The ability of β-lactam antibiotics, including penicillins, to covalently modify proteins is well established, and the prevailing hypothesis for T cell activation in adverse reactions to penicillins is the presentation of immunogenic penicillin-modified self-peptides by MHC molecules. We have recruited a cohort of penicillin allergic patients from The Alfred Hospital Allergy Clinic (n=10), who have experienced a range of adverse reactions to penicillins including amoxicillin. In preliminary studies we have isolated drug responsive T cells from a patient who experienced an accelerated response to amoxicillin that are activated via the MHC class I molecule HLA-A*02:01. Using a B-lymphoblastoid cell line derived from this patient, we are exploring the peptides presented by HLA-A*02:01 of untreated and drug treated cells via an immunoaffinity purification, LC-MS/MS workflow. Evidence for drug haptenated peptides will be discussed. These analyses will form the foundation of a broader characterisation of potential drug-induced T cell epitopes across the patient cohort.