Two patients with type I Glanzmann's thrombasthenia and 20 kindred of these patients belonging to 2 families of the Manouches gipsy tribe have been studied. Quantitative measurements of platelet membrane glycoproteins GP IIb and GP IIIa have made it possible to classify the patients into normal, thrombasthenic or carriers of the thrombasthenic abnormality. We have examined several red cell alloantigens and antigens of the major histocompatibility complex. These studies have shown that: 1) type I Glanzmann's thrombasthenia (GP IIb and IIIa abnormality) segregates independently of Ss and Fy systems and the A, B, C, Bf, C2 and C4 loci of the HLA complex; 2) a rare hemolytically inactive C4 variant segregates in these families but is not associated with the GP IIb and IIIa abnormality.