Quantitative relations between the group-specific and the type-specific components of the hexons of adenovirus type 2 and 5 were studied by means of FITC-conjugated Fab-fragments of antibodies directed against type 2 and type 5 hexons. From the sedimentation constant of the complexes of hexons and Fab in the region of excess of Fab we conclude that there are at least 20 determinants on the hexon. Half of these are type-specific and the others are group-specific. Both components of the type 2 hexon consist of equal parts of carbodiimide sensitive and carbodiimide resistent determinants.