Nucleic acid hybridization is extremely important in viral hepatitis research and the newly emerging techniques are now becoming an indispensable resource also in the diagnostic laboratory. This paper provides information on methods currently used for detection of viral nucleic acids with special emphasis on the importance of hepatitis B virus DNA in the serum. Herein, we describe the procedures for preparation and labeling of DNA probes and the principles that regulate dot, slot and Southern blot hybridization. Advantages and shortcomings of filter hybridization are discussed together with the alternative approaches to it. Finally, a collection of laboratory protocols is presented in Appendix #1.