Two monoclonal antibodies (MAbs) specific for two distinct epitopes on the human adenovirus type 1 (AV1) hexon were used to determine the subcellular localization of hexon epitopes in the infected HEp-2 cells by indirect immunofluorescence. On the basis of cross-reactivity pattern of MAbs, presumably one of the epitopes is genus specific and the other should be intertype specific. The epitopes, i.e. the adenovirus hexons could be detected throughout the cell and could display different accumulation forms. Fluorescence appeared either in the cytoplasm only or both in the nucleus and the cytoplasm. In the cytoplasm the hexons could be found in diffuse or perinuclear distribution or accumulated into discrete spots. In the nucleus they formed granules or clusters or were diffusely distributed causing a bright fluorescence of the whole nucleus. The different accumulation forms appeared at the same time in different cells of a culture, but in one given cell the fluorescence always appeared first in the cytoplasm.