From the ESR spectra of aqueous dispersions of synthetic glyceryl monostearate (spin labelled at C-12) a critical micelle concentration of 30 mumol/l at room temperature was obtained, which agrees with that deduced from surface tension measurements. At monoglyceride concentrations smaller or larger than the critical micelle concentration, the monomers show increased motional restriction with increasing molar ratio of beta-lactoglobulin to monoglyceride up to a value of 10, as determined from calculated rotational correlation times; A similar progressive interaction was deduced from spectral changes observed on equimolar dispersions of beta-lactoglobulin and monoglyceride on raising the temperature to 55 degrees C at which the protein and monoglyceride coprecipitate. The relevance of these finding for non-labelled monoglyceride dispersions is indicated by the similarity of the pH-dependent flocculation behaviour of labelled and non-labelled monoglycerides, both in the absence and presence of beta-lactoglobulin; In addition, proton magnetic resonance and mechanical stability measurements suggest that spin-labelled glyceryl monosterate behaves analogously to non-labelled glyceryl monooleate.