Summary. Hepatitis GB virus C/hepatitis G virus (HGV) is an RNA virus, which appears to be transmitted by parenteral exposure to contaminated blood and blood products, and may be associated with clinical hepatitis in humans. The prevalence of HGV was investigated in hepatitis C virus (HCV)-infected patients, and (HCV)-contaminated immune globulin intravenous products (IGIV). manufactured prior to the introduction of viral inactivation processing, and in recipients of these lots. Nested primers, specific for the 5′ non-coding region of HGV, were designed and used to test 100 chronic HCV patients. 10 HCV RNA-positive IGIV lots and 36 of the recipients of these products. Hepatitis G virus specificity of the polymerase chain reaction (PCR) products was confirmed by sequencing a number of the amplified products and comparing the results with the published prototype sequence for HGV RNA. HGV RNA was detected in 23 of the 100 (23%) HCV-infected patients. The level of alanine aminotransferase (ALT) was lower in HCV-HGV coinfected patients than those with HCV infection alone. Hence, the severity of HCV infections is not influenced by HGV. Two of the 10 (20%) IGIV lots tested positive for HGV RNA; however, none of the serum samples from recipients of IGIV contained detectable HGV RNA although many were infected with HCV. This suggests that the transmission of HGV RNA from IGIV to the recipients is less efficient than that seen for HCV.