A study of a family with a propositus suffering from classical thrombasthenia type I has shown that the new immunochemical methods detect heterozygotes with high reliability. There was no overlapping between heterozygotes and normals, and the concentration of the glycoprotein IIb-IIIa-complex is remarkably constant around 50–60% in the heterozygotes. Furthermore, heterozygotes as a group show an increased bleeding tendency.