Summary: The growth of four strains of vaccinia virus in the epithelial cells of the chorioallantois of the chick embryo was found to occur in a step-wise fashion. Immediately following the initiation of infection there was a sharp reduction in the amount of virus in the membrane, and no increase was detectable for approximately 8 hours. At this time virus could be released from the cells, and it did not increase further in concentration until 16 hours following the initiation of infection. After the completion of the second infectious cycle, virus was found in higher concentration in the chorioallantoic membrane. Vaccinia hemagglutinin was detected for the first time at about 16 hours following the inoculation of virus. At this time its presence was correlated with a second increase in viral concentration. It has been assumed that vaccinia hemagglutinin was released along with virus at 8 hours, but in such minute amounts that it could not be measured by the technics employed.