Varicella virus antigen was prepared and used in skin testing of 75 children one to 17 years old. Results of skin tests were analyzed in relation to history of varicella, presence of absence of neutralizing antibody, and susceptibility to clinical varicella. All 22 children with no history of varicella and no neutralizing antibody were negative in the skin test when an erythematous change of greater than or equal to 5 mm in diameter was taken as a positive result. Of the 53 children with neutralizing antibody, only three were negative in the skin test. The skin test was applied clinically in an institution for mentally retarded children immediately after varicella infection occurred in a patient. A total of 65 patients were examined by the skin test, and 24 children with negative results were given live varicella virus vaccine. As a result, no spread of varicella was observed except for three cases, which developed two weeks after onset of illness in the index case. Thus the skin test seems clinically useful in the determination of susceptibility of individuals to clinical varicella.