A 24-year-old woman with lupus-like serologic abnormalities had immune thrombocytopenia that resolved after splenectomy, but increased quantities of platelet surface IgG persisted. Three years later, during the 36th week of her first pregnancy, gamma globulin (400 mg/kg daily for 5 days) was administered intravenously to decrease the risk and/or severity of immune thrombocytopenia in her infant. The infusion produced marked but transient elevations of maternal concentrations of serum IgG and quantities of monocyte surface IgG, but no significant changes in Fc receptor-mediated rosetting of peripheral blood monocytes with antibody-sensitized platelets occurred. Modest increases in quantities of platelets and plasma platelet-specific IgG were demonstrated. The infant, delivered by cesarean section 2 days after the end of the infusion, had a normal platelet count; cord blood had a normal concentration of serum IgG, but an elevated quantity of platelet surface IgG (by comparison with values for normal adults). Infant values of plasma platelet-specific IgG, monocyte surface IgG, and monocyte/platelet rosettes also were within the range of normal for adults. Anticytomegalovirus antibody was present in large amounts in the gamma globulin infused, first appeared in maternal serum after therapy, and was detected in cord serum. The significance of these observations to the management of immune neonatal thrombocytopenia is discussed.