Cho, C. T. (University of Kansas School of Medicine, Kansas City, Kansas 66103), C. R. Bolano, P. S. Kamitsuka and H. A. Wenner. Methisazone and monkey pox virus: Studies in cell cultures, chick embryos, mice and monkeys. Amer. J. Epid., 1970, 92: 137–144.—-Methisazone suppressed plaque development in tissue cultures infected with monkey pox and vaccinia viruses. Survival times of chick embryos infected with vaccinia virus were consistently greater than were those of chick embryos infected with monkey pox virus (MPV). Temporary prolongation of life was obtained among mice treated with methisazone and challenged intraperitone ally with MPV. No protection was afforded mice challenged intracerebrally. Death invariably occurred in treated chick embryos and in mice. Monkeys treated with methisazone developed monkey pox; excepting for equivocal modification of the clinical features of disease, immunologic and pathogenetic features of infection were indistinguishable from those of infected control monkeys. Within limitations of experimental designs in these living hosts, methisazone failed to prevent or to unequivocally modify infection induced by MPV.