A soluble varicella virus antigen for skin testing was developed using culture fluid from varicella-zoster virus-infected human diploid cells. The culture fluid was centrifuged at a high speed (100,000 g for 2 hr) to remove viral particles, and the supernatant, after heating at 56 C for 30 min, was used as the soluble skin test antigen. No virion or nucleocapsid was found by electron microscopy, and very little 3 H-labeled DNA in the soluble antigen hybridized with viral DNA. By immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and fluorography, several viral glycoprotein bands were detected in the soluble antigen. In a comparative skin test on 10 medical students and 10 children using soluble and crude antigens prepared from culture fluid or infected cells, the skin reaction to the soluble antigen tended to be less than that to the crude antigens, but the former reaction correlated well with a history of varicella and the presence or absence of virus-neutralizing antibodies.