Effects of time, temperature, pH and stabilizers (i.e. medium) on inactivation of lipid-enveloped model viruses, Semliki Forest and vesicular stomatitis viruses in the production process of intravenous immunoglobulin were investigated on a laboratory scale. The lowering of pH, the raising of temperature and the increasing of incubation time improved the inactivation effect. However, small changes in pH and stabilizer concentrations did not influence the results. Inactivation was not linear and a clear tailing off could be seen. Therefore, for complete virus inactivation incubation times longer than 20 h are necessary. Inactivation took place much more rapidly in intravenous immunoglobulin solution than in intramuscular immunoglobulin solution. Processing steps such as freeze-dying in the presence of ethanol or storage of intramuscular immunoglobulin in the liquid state at pH7 only partially inactivated these viruses.