A genomic fragment of hepatitis B virus encoding the surface antigen (HBsAg) was inserted into the proviral genome of Moloney mouse sarcoma virus (MSV), obtained from the mouse cell line G8 ‐124. The recombinant DNA was introduced into NIH 3T3 mouse fibroblasts. Cells, morphologically transformed by the oncogene of MSV (v‐mosM) were selected, established as cell lines and tested for expression of HBsAg. An expression level of up to 4.5 micrograms/10(7) cells/day was detected.