The effect of divalent cations on the susceptibility of mammalian cells to infection by viruses has been the subject of several investigations. Calcium enhances poliovirus plaque formation on rabbit embryo kidney cells (Cooper, 1961) and magnesium increases the susceptibility of monkey kidney cells to poliovirus (Wallis & Melnick, 1962). Plaque formation by certain rhinoviruses in HeLa cells is enhanced by higher concentrations of calcium and magnesium in the overlay medium (Fiala & Kenny, 1966; Stott & Tyrrell, 1968). The increased susceptibility of monkey kidney cells to poliovirus results from the earlier release of virus from infected cells (Wallis & Melnick, 1962), and the release of rhinovirus from HeLa cells is also greatly enhanced in the presence of high levels of MgCl2 (Fiala & Kenny, 1967). This communication provides evidence that MgCl2 also enhances plaque formation by human adenoviruses on HeLa cell monolayers and that the effect is due to an increase in the rate of virus release from infected cells.