The mechanisms underlying the modulation of Natural Killer (NK) cell functions by intravenous immunoglobulin (IVIg) are poorly understood. Using an ex vivo whole blood assay system we demonstrate that IVIg suppresses NK cell cytotoxicity. This was paralleled by IVIg-induced degranulation of CD56(bright), CD16(positive) NK cells, reduced expression of CD16 and elevated IFN gamma release. To assess whether these findings also occur in vivo we analyzed whole blood before and after IVIg therapy of patients. Following IVIg treatment the number of NK cells in peripheral blood dropped significantly. We observed reduced CD16 expression, elevated IFN gamma-amounts in plasma, reduced NK cell cytotoxicity, and granzyme B release into the plasma, confirming our in vitro data. These effects on the functions of NK cells describe a novel immunomodulatory effect of IVIg. The in vitro assays employed here could represent informative test systems to monitor effects of in vivo IVIg treatment at an individual level.