The efficacy of zoster immunoglobulin (ZIG) in preventing varicella was studied among patients in a high-risk group. 173 non-immune patients were observed after exposure to varicella-zoster (VZ) virus and subsequent administration of ZIG. For 138 patients (80%) no sign of varicella was recorded, 16 patients (9%) had a subclinical infection and 19 patients (11%) developed varicella. 12/19 patients with varicella contracted a very mild disease (less than 20 pocks, negligible fever), 5 got mild or normal disease and 2 children, both with acute lymphatic leukemia, developed more pronounced symptoms. Three patients protected by 0.15 ml ZIG/kg body weight after heavy exposure to VZ virus, were not protected at a second exposure 2 weeks later. In an enlarged study group of high-risk patients where 52 patients receiving ZIG developed varicella, the mean incubation period for 42 patients was 21 days. Leukemic patients were found to have a higher frequency of clinical varicella, more pronounced symptoms and a slightly longer incubation period than other high-risk patients. VZ specific antibody titers were compared for various immunoglobulin preparations and found to be 30 times higher in zoster immunoglobulins than in normal immunoglobulins.