Members of the oxytocinase sub-family of M1 aminopeptidases (ERAP1, ERAP2 and IRAP) play important roles in both the adaptive and innate human immune responses. Their enzymatic activity can contribute to the pathogenesis of several major human diseases ranging from viral and parasitic infections to autoimmunity and cancer. We have previously demonstrated that diaminobenzoic acid derivatives show promise as selective inhibitors for this group of aminopeptidases. In this study we have thoroughly explored a series of 3,4-diaminobenzoic acid derivatives as inhibitors of this class of enzymes, achieving sub-micromolar inhibitors for ERAP2 (IC50=237 nM) and IRAP (IC50=105 nM). Cell-based analysis indicated that the lead compounds can be effective in down-regulating macrophage activation induced by lipopolysaccharide and interferon-?, as well as cross-presentation by bone marrow-derived dendritic cells. Our results indicate that this class of inhibitors may be useful for the targeted down-regulation of immune