For all positive-sense RNA viruses, replication occurs on the surface of host cell membranes. Ultrastructural studies have shown that many of the viruses induce extensive proliferation and rearrangements of intracellular organelles. Research in a number of laboratories has suggested a variety of hypotheses for their origins. Hepatitis C virus, which has become a major worldwide health concern, is a positive-sense RNA virus; however, HCV research has been restrained by the lack of a tissue culture system. By analogy with all other positive-sense RNA viruses, it is presumed that HCV RNA replication occurs on the surface of host cell membranes. The goals of this research are to gain a better understanding of the interactions of HCV proteins with host cell membranes. Immunofluorescence microscopy and cellular fractionation studies as well as ultrastructural studies will be utilized to determine which viral and cellular proteins are associated with HCV induced membrane structures. By examining subsets of HCV nonstructural proteins, these studies may also illustrate the mechanism by which HCV proteins are recruited to complexes that may be critical for RNA replication.