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HLA-DR is the most commonly expressed and likely the most medically important human MHC class II, antigen presenting protein. In a normal immune response, HLA-DR binds to antigenic peptide and the HLA-DR/peptide complex binds to a T-cell receptor, thus contributing to T-cell activation and stimulation of an immune response against the antigen. When foreign antigen is not present, HLA-DR binds endogenous peptide which, under normal conditions does not stimulate an immune response. In most cases, the human peptide is CLIP, but a certain percentage of HLA-DR molecules will be present at the cell surface with other human peptides. We have recently shown that cell surface, CLIP/HLA-DR ratios are a measure of peptide heterogeneity, and in particular, changes in CLIP/HLA-DR ratios represent changes in the occupancy of HLA-DR by other, endogenous peptides. For example, treatment of cells with the HDAC inhibitor, Entinostat, leads to an upregulation of Cathepsin L1 and replacement of Cathepsin L1 senstitive peptides with HLA-DR binding, Cathepsin L1 resistant peptides, an alteration that can be at least partially assessed via assessment of CLIP/HLA-DR cell surface ratios. Here we assay for CLIP/HLA-DR ratios following treatment of immortalized B-cells with a variety of common drugs, almost all of which indicate significant changes in the CLIP/HLA-DR ratios. Furthermore, the CLIP/HLA-DR ratio changes parallel the impact of the drug panoply on cell viability, suggesting that alterations in the HLA-DR peptidome are governed by a variety of mechanisms, rather than exclusively dependent on a dedicated peptide loading process. These results raise questions about how FDA approved drugs may affect the immune response, and whether any of these drugs could be useful as vaccine adjuvants?