Abstract HeLa cells singly infected with adenovirus were irradiated with ultraviolet light at various times during the viral latent period. The experimental data show: (1) Extracellular virus was more resistant to irradiation than were the infective centers. (2) The rate of inactivation of the capacity of the cells to become infective centers was the same as that of the infective centers. (3) There was no change in the rate of the inactivation of the infective centers during the entire viral latent period. Increased resistance to irradiation was accompanied by the appearance of newly synthesized virus. It was also found that multiply infected cells yield different data, apparently because they contain at all times during the latent period uneclipsed virus which is infectious when released from the disrupted cells.