The incidence and severity of varicella following a close family contact were evaluated in children with neoplastic diseases who received prophylaxis either with commerical gamma globulin or with zoster immune plasma, as compared to patients who did not receive any prophylaxis. In the untreated group, all 14 patients developed varicella, complicated by 1 case of encephalitis and 2 cases of fatal pneumonia. In the group of 17 patients who received 0.6-1.2 ml/kg body weight of gamma globulin, 16 developed varicella, complicated by pneumonia in 2 cases, with 1 death. In the third group of 27 patients who received 10 ml/kg body weight of zoster immune plasma (ZIP), obtained from healthy adults convalescing from herpes zoster, there were only 8 cases of varicella, all very mild. Thus, prophylaxis with ZIP significantly reduced the incidence of clinical varicella (p less than 0.01) and attenuated the severity of its course.