The binding of monoacylglycerides of long-chain fatty acids to human serum albumin has been examined using monooleoylglycerol as the ligand. Binding was investigated using changes in tryptophan fluorescence and also the displacement of a variety of well-studied fluorescent ligands from human serum albumin (HSA). Monooleoylglycerol caused a decrease in fluorescence from tryptophan-214 when measured at 350 nm while oleic acid had no effect on fluorescence at this wavelength and did not compete with monooleoylglycerol. In contrast, oleic acid caused an increase in fluorescence at 330 nm whereas monooleoylglycerol did not affect fluorescence intensity at this wavelength. These results suggest that these two ligands do not bind to the same site on HSA. From competition studies using dansylglycine, dansylsarcosine, 11-(dansylamino)-undecanoic acid and 1-anilino-8-naphthalenesulfonic acid it was proposed that monooleoylglycerol binds at the dansylsarcosine site (site II) of HSA. Monooleoylglycerol was a competitive inhibitor of dansylsarcosine binding with a Kd of about 2.5 microM whereas oleic acid was not competitive with dansylsarcosine binding.