Summary Functional and biochemical studies of platelets from human Chédiak-Higashi syndrome (CHS) are scarce and/or incomplete. In the present report, the aggregation response to a variety of inducers of platelet aggregation, the content of the dense granule constituents ATP, ADP, serotonin and calcium, the secretion of ATP, ADP, and calcium induced by thrombin, the total content of magnesium, the incorporation of 14C-adenine in the cytoplasmic pool of adenine nucleotides, as well as the content of intracellular cyclic-AMP, have been quantitated in six patients with CHS. Furthermore, data is presented on the kinetics of uptake of radiolabelled serotonin and its storage in human CHS platelets. An abnormal aggregation behaviour was found in all patients. However, the response of CHS platelets to the different inducers studied did not show a uniform pattern. The total content and the maximal amounts of the dense granule constituents secretable by thrombin were greatly decreased in all six patients. Total magnesium content was similar to that of normal platelets. The ATP/ADP ratio was higher than in controls. Uptake of radiolabelled serotonin by CHS platelets closely followed the uptake by normal platelets; during the first 2–3 min, however, incorporation of the amine by CHS platelets came rapidly to a plateau which contrasts with the steady, linear increase in uptake found in controls. CHS platelets loaded with radiolabelled serotonin and gel-filtered, showed a spontaneous release of radioactivity not observed in normal platelets under the same conditions. The cyclic-AMP content of CHS platelets was similar to that of normals. In contrast to platelets from patients with storage pool disease, the secretable calcium from CHS platelets represents a 67% of total platelet