The infectivity of cytomegalovirus (CMV), strain Davis, was inactivated by 4-nitroquinoline 1-oxide (NQO). A series of survival curves indicates that the rate of inactivation was directly dependent on the concentration of NQO over a range of 5 to 200 microgram/ml. At concentrations of I microgram/ml or less, inactivation of virus stock was not observed and at concentrations in excess of 200 microgram/ml, the cellular toxicity of residual NQO prevented quantification of the relatively low surviving infectivity. At a concentration of 200 microgram/ml NQO or less, the loss of virus infectivity could be clearly shown to result from the interaction of NQO with virus and not with cells, since the addition of similar doses of NQO to assay cells simultaneously with virus did not adversely affect the sensitivity of the assay cells to measure virus infectivity. Similarly, the dimethylsulphoxide carrier at concentrations of 5% or less was shown to have a negligible effect on both virus infectivity and on the sensitivity of human skin muscle cells to assay virus infectivity. NQO inactivation of virus infectivity appeared to depend very little on white light, since the kinetics of inactivation in the presence and in the absence of white light were similar.