Abstract A fluorimetric method for measuring the serotonin concentration in tissues has been adapted for the determination of this amine in platelets. The characteristics of the method were assessed and found to be satisfactory. A range of platelet serotonin was established for normal children and some cases of phenylketonuria and histidinaemia were examined. In view of the results found for patients with these disorders the effect of high concentrations of phenylalanine and histidine in the method was investigated. Phenylalanine was found to add considerably to the fluorescence, but interference by histidine was not found to be significant.