Antibody to varicella-zoster virus was measured in Vero cell cultures by a conventional and an enhanced plaque neutralization test. In the latter, heterologous antibody to human immunoglobulin was added to the virus-serum mixture to potentiate the neutralization reaction. The antibody to human immunoglobulin enhanced the sensitivity of the neutralization test seven- to 107-fold as compared with the conventional procedure. The test was highly reproducible in that individual serum titers fluctuated in a twofold range on repeated testing. A prozone phenomenon was observed in the enhanced neutralization test in low-titered sera, but not with high-titered sera or with specimens of cerebrospinal fluid. The specificity of the test was confirmed by the detection of seroconversion in recipients of live varicella virus vaccine.