A requirement for a minimal threshold level of moisture in order for efficient virus inactivation to occur during dry heat treatment of freeze-dried coagulation factor concentrates is described. Techniques used to determine moisture content during heating were Loss on Drying and Karl Fischer. The Loss on Drying was suspected to have occasional errors as a result of sample preparation being influenced by interference from atmospheric moisture. Therefore, a non-invasive, non-destructive method for determination of residual moisture content using near infrared spectrometry(NIR) was developed for freeze-dried antihaemophilic factor (AHF). Calibration equations were determined against Loss on Drying and Karl Fischer assay methods and these equations evaluated for the predictive efficiency. Both Loss on Drying and NIR were used to evaluate the effect of moisture content on the efficiency of virus inactivation by dry heat at 80 degrees C. A minimum level of moisture of greater than 0.7%, as determined by Loss on Drying, was necessary for a virus reduction in the magnitude of 4 log(10) for hepatitis A virus, porcine parvovirus and pseudorabies virus. (C) 1998 The International Association of Biological Standardization.