The effect of cytomegalovirus (CMV) infection on the cell surface expression of the adhesion molecules LFA-3 and ICAM-1 was studied by flow cytometry using human embryo lung fibroblasts. Our results demonstrated a marked increase in the expression of these molecules from days 1 to 5 post-infection. Peak expression of LFA-3 and ICAM-1 on the surface of the infected cell occurred at 2 days post-infection, when LFA-3 was twofold, and ICAM-1 threefold, greater than the level observed on uninfected fibroblasts. In contrast, parallel studies on the expression of class I HLA confirmed our previous findings that CMV induces a down-regulation of this molecule on the surface of the infected cell, and further demonstrated that at the time of maximum increase in LFA-3 and ICAM-1 expression, class I HLA expression was only 46% of the uninfected cell level. Immunofluorescence and confocal scanning laser microscopy revealed markedly enhanced expression of LFA-3 in infected cells, with accumulations in discrete granules in the perinuclear area, contrasting with the diffuse cytoplasmic distribution of this molecule in uninfected fibroblasts. ICAM-1 was found to be highly localized at the cell membrane of infected cells, whereas a diffuse cytoplasmic distribution was observed in the uninfected cell. The mechanism of the up-regulation of adhesion molecule expression on the CMV-infected cells remains to be determined; however cytokines known to up-regulate ICAM-1 were not detected in the culture supernatant of infected cells. The effects of CMV infection on the adhesion of peripheral blood leucocytes to fibroblasts is described in the accompanying manuscript (p. 413).