Antibody titers to varicella-zoster virus were measured in varicella-susceptible immunocompromised children 48 h after they received either one of two lots of zoster immune globulin (ZIG) or a selected lot of immune serum globulin (ISG). Globulin was given to modify varicella in these children after exposure to varicella or zoster. Indirect immunofluorescence antibody titers (FAMA) of children after receipt of globulin ranged from less than 1:2 to 1:32. Geometric mean FAMA titers were highest after 1.2 ml of ISG per kg (FAMA titer 1:128) and 0.16 ml of ZIG lot A per kg (FAMA titer 1:1,024). Selected batches of ISG titering 1:128 or greater by FAMA, at a dose of 1.2 ml/kg, may be used to attempt to modify varicella in susceptible high-risk individuals when ZIG is not available.