i.v. IgG therapy was used to treat thromboneutropenia in 2 male neonates of mothers with SLE. The mothers of both neonates were thrombocytopenic but not neutropenic during the last 3 months of their pregnancies. Anti-platelet and anti-neutrophil IgG directed against platelets and neutrophils from the neonates were detected in the sera of both mothers. The platelet and neutrophil counts increased significantly with i.v. IgG therapy in both neonates. i.v. IgG therapy is a safe and effective regimen for passive immune thromboneutropenia.