Soluble hexon of type 1 adenovirus was highly purified with different techniques and dialysed against 0.5 M acetate buffer. With this procedure tetrahedral crystals were produced from the soluble hexon capsomers of the virus capsid. Electron microscopic observation of the crystallization process, revealed the development of dense two-dimensional "crystal sheets" following dialysis, in homogeneous hexon preparations containing single hexons. No such formations were observed so far with other types. The occurrence of two-dimensional crystals decreased proportionally to the appearance of three-dimensional crystals, which refers to their possible role in the mechanism of three-dimensional crystal formation.