A process produces DNA corresponding to that of the DNA of the virus of B hepatitis. It comprises cloning in bacteria the latter DNA, previously repaired by means of the corresponding precursor nucleotides in the presence of a polymerase. Vectors contain the cloned DNA in their genomes. The cloned DNA is useful as a probe for detecting the presence of the virus of B hepatitis in biological samples, particularly blood or plasma. Its expression in bacteria provides a hybrid protein containing a protein fragment having vaccinating properties against hepatitis B. Nucleic acid of reduced size and a vector containing the nucleotide sequence of which DNA codes an immunogenic peptide sequence capable of inducing the generation of antibodies to the virus of viral hepatitis B. It comprises totally or partly the sequence of nucleotides represented in FIG. 9A. Application to the production by cloning in a bacterium of an immunogenic protein immunizing against hepatitis B, or application to the obtention of probes for the diagnosis of the presence of Dane particles in the serum.