Our investigation indicates that pretreatment of human immunoglobulin for the elimination of the anticomplementary activity is associated with a loss of activity, the extent of which depends on the type of treatment applied. Laboratory preparations of human IgG were tested in a mouse protection assay using influenza A2-Taiwan virus, tetanus toxin and Salmonella typhimurium as the challenge. There was a 7-28% reduction in efficacy in an intravenous 7S preparation in comparison with an untreated 7S IgG. F(ab')2 fragments showed a 24-65% and Fab fragments an 80-100% reduction in efficacy. Two commercial human 7S products showed approximately 90% efficacy in the Salmonella assay; a commercial, pepsin-treated preparation showed 65-74% efficacy when compared with untreated 7S IgG.