A specific radioimmunoassay has been developed for human plasma motilin. No cross reaction was seen with other gut hormones and only a single major peak of motilin immunoreactivity was found in plasma and gut extracts after gel chromatography. The motilin content of the human duodenum was 165.7+/-15.9 pmol/g wet weight and jejunum 37.5+/-2.8 pmol/g with negligible amounts in the rest of the gut. Fasting plasma motilin concentrations in 100 healthy subjects ranged from less than 5 to 300 pmol/l (mean 72 pmol/l).