Human anti-chickenpox immunoglobulin (zoster immune globulin, ZIG) was largely ineffective in preventing infection in forty-three high-risk contacts of chickenpox. Twenty-nine of these non-immune infants and children who had been in close contact with cases of varicella became infected, and symptoms developed in twenty-four. Since ZIG may modify chickenpox it should continue to be given to high-risk contacts until the availability of a simple and sensitive test makes it possible to identify those who are susceptible. However, ZIG is not necesary for infants whose mothers have chickenpox or zoster five or more days before delivery, since all such infants (eleven) had varicella-zoster antibody at birth.