THE development of a plaque technique for titrating vaccinia virus on monolayers of chick embryo cells without an agar overlay1 has furthered the investigation of cell-virus interaction in this system and has produced evidence of an ‘eclipse phase’ under certain experimental conditions. The essential prerequisite was that the inoculum, consisting of an elementary body suspension prepared from a rabbit skin-passaged strain of vaccinia by the method of Hoagland, Smadel and Rivers2, was treated ultrasonically before use. Following the infection of monolayers by contact with this inoculum for 5 min. there was a rapid loss of infectivity of adsorbed virus, estimated to be probably about 30–40 per cent of the amount adsorbed, occurring within 10 min. from the first contact between virus and cells. Thereafter the rate of loss was slower, a further 70 per cent of the virus present at the end of the first 10 min. disappearing during 4–6 hr. The titre of virus in the culture began to rise at 7–8 hr. due to production of new virus.