Human intravenous immunoglobulins prepared by the cold ethanol fractionation technique of Cohn are considered safe with respect to infectivity. However, there have been several instances of transmission of both hepatitis B and non-A,non-B hepatitis viruses after administration of intravenous immunoglobulins. To determine the prevalence of hepatitis C virus antibody in intravenous immunoglobulins and protein preparations, 30 commercially available products were tested. Using the Abbott enzyme immunoassay for hepatitis C virus antibody, 27 of 30 (90%) immunoglobulins tested positive. The Ortho immunoassay showed that 28 of 30 (93%) were positive, with one discordant result between the Ortho and Abbott assays. An antigen-blocking or neutralization test (Abbott) confirmed the results of the Ortho assay. Bovine, sheep, goat, and horse sera also were tested before and after isolation of animal immunoglobulins. All results on the animal sera were negative, indicating that the fractionation process did not produce false-positive results. The high prevalence rate of hepatitis C virus antibody in intravenous immunoglobulins has important implications for follow-up of recipients, selection of serum donors, and implementation of anti-hepatitis C virus testing.