Water extracts of garlic, deodorized garlic powder, and onions, but not leeks, were found to significantly (p < 0.05) reduce the in vitro formation of N-nitrosomorpholine (NMOR), a known liver carcinogen. Addition of increasing quantities (20, 40, and 80 mM) of S-allyl cysteine (SAC), a water-soluble compound in processed garlic, depressed NMOR formation by 16%, 27%, and 43%, respectively (p < 0.05). The ability of SAC to block NMOR formation decreased as the NaNO7 and morpholine concentrations increased. SAC and its non-allyl analog S-propyl cysteine effectively blocked NMOR formation. SAC and S-propyl cysteine were less effective than isomolar cysteine in reducing NMOR formation (p < 0.05). The oil-soluble sulfur compounds diallyl disulfide (DADS), dipropyl disulfide, and diallyl sulfide were ineffective inhibitors of NMOR generation (p > 0.05). SAC and DADS reduced the mutagenicity of NMOR in Salmonella typhimurium TA100 (p < 0.05). SAC at 70 mumol/plate reduced the number of histidine revertants per plate by 51% (p < 0.05), whereas DADS at 0.12 mumol/plate reduced mutant colony number by 76% (p < 0.05). SAC and DADS were more effective than isomolar cysteine in reducing NMOR mutagenicity (p < 0.05). The ability of sulfur compounds in garlic and onions to depress nitrosamine formation and bioactivation in these studies is consistent with epidemiologic evidence that higher intake of allium plants is associated with a reduction in the risks of some cancers.