Varicella-zoster (V-Z) virus was first cultivated in tissue culture by Weller & Stoddard (1952). Subsequently the fluorescent antibody technique was used by Weller & Coons (1954) to demonstrate that antibodies to the virus are present in convalescent chickenpox and zoster sera, but few neutralization tests have been done with the agent because of the difficulty in obtaining virus in a cell-free state from most tissue culture systems. Weller & Witton (1958) showed that the ability of intact infected human embryonic fibroblasts to transmit virus to fresh tissue culture monolayers could be partially neutralized by antiserum if the antiserum was incorporated in the medium of the inoculated tissue cultures. The cytopathic effects of the virus were not suppressed completely but the number of foci of infection was greatly reduced and the foci themselves were altered in appearance. Taylor-Robinson (1959) described neutralization tests in which vesicle fluid taken directly from patients was used as a source of cell-free virus. This system gave satisfactory results but its application was limited by the difficulty of obtaining sufficient vesicle fluid. The production of cell-free virus from primary human thyroid cells (Caunt, 1963) provides a more convenient laboratory source of cellfree virus and this paper describes the method we have used for detecting V-Z virus neutralizing antibody in the sera of chickenpox and zoster patients and in pools of human gamma-globulin. Kapsenberg (1964) and Ross, Subak Sharpe & Ferry (1965) showed some crossreactions between Herpes simplex (HS) and V-Z virus in complement-fixation (CF) tests so we also tested most of our sera with HS virus to see whether any crossneutralization could be demonstrated. The paired sera were also tested for CF antibody to both V-Z and HS antigens.