Summary The interaction of sheep, horse, pig, human and guinea-pig whey proteins with fatty acids has been studied. Using gel filtration and autoradiography, it was found that sheep β -lactoglobulin and serum albumin from all species had the ability to bind fatty acids in vitro . Sheep β -lactoglobulin, isolated from milk, had ˜ 0·5 mol fatty acids bound per mol monomer protein, and albumin from sheep, horse and pig contained ˜ 4·5, 2·9 and 4·7 mol fatty acids/mol protein respectively. However, β -lactoglobulin from horse and pig milk had neither fatty acids physiologically bound nor the ability to bind them in vitro . Albumin was the only whey protein detected with bound fatty acids in these species as well as in human and guinea pig. This suggests that the ability of ruminant β -lactoglobulin to bind fatty acids was not shared by the same protein of non-ruminants.