The platelet levels of serotonin and the amino acids aspartic acid, glutamine, glutamic acid and gamma-aminobutyric acid were measured in 18 drug-free autistic (DSM-III criteria) and 14 age-matched healthy children. Serotonin was significantly increased while the amino acids aspartic acid, glutamine, glutamic acid and gamma-aminobutyric acid were significantly decreased in comparison with the controls. It is suggested that the decline of the amino acids in platelets from autistic children represents a biochemical marker related to infantile autism.