Genetic variability of the Hepatitis B virus (HBV) strongly impacts the natural history of infection and the efficiency of diagnosis, vaccination and treatment. A genotypc assay able to provide genetic information on all HBV genes would be a very important tool for clinicians and epidemiologists. The DNA-Chip technology has proved to be powerful and convenient for the diagnosis of infectious diseases due to bacteria and viruses. We have designed a genotyping assay based on polymerase chain reaction and DNA-Chip for HBV Starting from a unique specimen, analysis of polymorphisms at 150 positions along the genome and 383 mutations is possible as well as the determination of the genotype. Preliminary experiments with plasmas from infected patients show that results obtained with this reagent are strongly correlated with those obtained with sequencing.