In a group of student volunteers vaccinated at the age of 13-17 years with vaccine produced by SmithKline Beecham Biologicals which contains the live attenuated strain of the Oka virus of varicella zoster the period of the protective effect of vaccination was assessed. In the course of 310,000 man-days of detailed investigation the authors revealed in 55 vaccinated subjects close exposure to a patient with varicella, and six of the vaccinated subjects contracted the disease after exposure. In the course of the subsequent three years no further case of varicella was recorded. Subjects vaccinated only by a single dose of the vaccine with low post-vaccination levels of specific antibodies contracted the disease.