A strain of varicella virus was isolated and easily maintained in HeLa cells. The infection could be passed with live cells only, practically all infectivity being destroyed by disruption of the cells with sonication or repeated freeze-thawing, even if precautions were taken to favour the infectivity of nucleic acids. Potent CF antigen was easily obtained from infected HeLa cultures. However, the high proportion of cross reactions in adult cases of herpes simplex hamper the diagnostic value of the CF test with varicella antigen.