A series of dihydrouracil derivatives has been synthesized and investigated for their in vitro inhibitory activity toward human leukocyte elastase (HLE) and cathepsin G (Cath G). Alkyl [sulfonyl(oxy)] uracils 1-2 were found to be efficient, time-dependent inhibitors of elastase (kobs/[I] M-1 s-1 values ranged between 480 and 8110). These compounds formed acyl enzymes that exhibited variable hydrolytic stability which appeared to be dependent on the nature of the R1 group (believed to be accommodated at the primary specificity site, S1). The acyl enzymes formed with cathepsin G deacylated rapidly, leading to a significant regain of enzymatic activity. In sharp contrast, the corresponding phosphorus compounds 3-4 were found to be potent, time-dependent irreversible inhibitors of HLE. Furthermore, the results of the structure-activity relationship studies suggest that the binding modes of compounds 1-2 and 3-4 may be different.