The hepatitis C virus (HCV) resource at Los Alamos (hcv.lanl.gov) provides access to multiple databases: one containing annotated sequences and the other a repository of immunogenic epitopes. They are derived from databases originally developed for HIV research (hiv.lanl.gov). HCV and HIV are RNA viruses with relatively compact genomes (around 10 kb) that are extraordinarily variable, both within and between hosts. This diversity requires methods to track and exclude variants from an individual infection or from epidemiologically related infections, and tools to analyse the variation. The HCV immunology database contains a curated inventory of immunogenic epitopes and information about their interaction with the host immune system, with associated retrieval and analysis tools. This interactive resource provides flexible retrieval tools for sequences, epitopes, clinical information, and meta-data, as well as utilities for scientific data analysis, to investigators with internet access and a web browser. This paper describes the types of data and the services that these databases offer, the tools they provide, and their configuration and use. Examples of applications to clonal analysis for drug-resistance mutations are shown.