Platelet factor-3 activity and availability, platelet phospholipids and their fatty acid and aldehyde composition were examined in patients with juvenile diabetes of long duration and in patients with maturity onset diabetes. Increased coagulant activity was found in platelet rich and platelet poor plasma from patients with juvenile diabetes, whereas plasma from patients with maturity onset diabetes reacted as the control group. Platelet rich plasma exposed to ADP, kaolin or freezing and thawing three times reacted not significantly differently from the controls. Increased amount of lipid-P was present in platelets from both patient groups. In patients with juvenile diabetes the increase was mainly caused by an increase of serine phos-phoglycerides and in the other patient group all main phos-pholipid fractions were increased. The phospholipid/protein ratio of platelets was not significantly different in the three groups. Only moderate changes were observed in the fatty acid pattern of the various phospholipids.