A selected ion monitoring method for the estimation of an anti-allergy compound isamaxole (N-butyl-N-(4-methyloxazol-2-yl)-2-methylpropanamide, LRCL 3950) in plasma has been developed. The method uses a stable analogue containing nine deuterium atoms as internal standard. The assay is sensitive to 2 ng ml-1 plasma and has been used to examine plasma levels of isamoxole after doses of the compound to animals and man.