Acetyl-6-formylpterin is an inhibitor of mucosal-associated invariant T (MAIT) cell activation. It dose-dependently increases the surface expression of MHC-related 1 (Mr1) in WT3 cells overexpressing Mr1 (WT3-m) following incubation with E. coli but does not activate MAIT cells cocultured with WT3-m cells when used at concentrations ranging from 0.23-15 µM. Acetyl-6-formylpterin is also a degradation product of folic acid and a precursor in the synthesis of 2’-hydroxy-7,8-dihydrofolate, which is a metabolite of para-aminosalicylic acid (PAS) in M. tuberculosis.