The inhibitory effect of chelerythrine, sanguinarine, fagaronine andsanguiritrine on the membrane proteases DPP IV and APN is described.Inhibition of these proteases by QBA in serum is suppresed by albumin.To explaine this phenomenon, the mode of QBA binding with albumin has beeninvestigated.Because of well documented role of DPP-IV/CD26 and APN/CD13enzyme activities in the processes of cell activation and differentiation,their inhibitors could be useful tools for fundamental enzymological studiesas well as for future practical applications.