Gamma-glutamyltransferase (GGT) activity in human platelet sonicates was 13.6 u/g of protein (range: 7.9–25.0) in 13 healthy, non-smoking, female volunteers; corresponding values in 16 males were: 20.3 (10.1–26.0). These values Mered significantly (p = 0.034). Platelet and serum GGT activity correlated significantly (p>0.04). Platelets seem to contain only the isoenzyme GGT 4. Part of this enzyme activity is in the form of aggregates or linked with membranes/proteins. This activity is released by Triton X-100 and trypsin and migrates as GGT 4. Serum GGT activity, a measurement in routine use, could be influenced by GGT released by platelets. It is therefore of interest that serum GGT activity can be increased in clinical conditions (e.g. myocardial infarction, diabetes, peripheral vascular disease) associated with platelet hyperactivity. Platelet GGT may influence intracellular S-nitrosoglutathione (a putative nitric oxide donor) levels. Potential associations between serum GGT activity and platelet function indices deserve investigation.