The analysis of UV spectra of human platelet particulates showed the existence of an evident protein component ordered in alpha-helical conformation in normal subjects. This ordered component was enhanced in type IIa hyperbetalipoproteinemia and, on the contrary, very low in a case of heterozygous familial hypobetalipoproteinemia. In the latter condition almost all proteins were indeed disordered as random coils. These data thus suggest that cholesterol significantly contributes to the control of molecular arrangement of platelet membrane proteins.