SUMMARYA technique for neutralization tests using varicella-zoster virus propagated in primary human thyroid cells is described. The level of neutralizing antibody following chickenpox does not usually exceed a titre of 1/40 and in adults many years after infection it may be very low. After zoster a much higher and more persistent antibody response occurs. Contact with chickenpox also produced a rise in neutralizing antibody in one out of the five patients tested. One case who had had chickenpox but not zoster had a high level of neutralizing antibody and the possible reasons for this are discussed. No cross-neutralization withHerpes simplexvirus was demonstrated but the rise in titre of complement-fixing antibody to HS occurring in herpetic subjects with chickenpox (Rosset al.1965) was confirmed. Two samples of human gamma-globulin were shown to have high levels of neutralizing antibody to V-Z virus and one was known to have been found effective clinically.We are indebted to Dr R. E. Hope-Simpson for the paired contact sera and also for many valuable discussions in the course of this work. We would also like to thank Dr D. Taylor-Robinson who collected the paired zoster sera, Dr R. M. Rawcliffe and Dr F. Whitwell who arranged the supply of thyroid tissue, and Dr G. C. Turner and Dr J. Eller for the samples of human gamma-globulin. We are grateful to Mrs M. Stewart for valuable technical assistance.