Fish lymphocystis disease viruses (FLDV) were isolated directly from lymphocystis disease lesions of various flatfish species and further purified. Subunits could be identified only after the purified virus was disrupted. In combination with different types of treatment, Nonidet-P40, dithiothreitol, proteases digestion and after ultrasonication and ultracentrifugation, the inner region of FLDV was studied. The purified virus was used for isolation of the virus nucleoid and for further study of the viral genome. Contour length measurements of 20 DNA molecules gave an average length of 40.44 +/- 3.2 micron. Lines of precipitation between isolated nucleoid material and FLDV-antibodies were shown by immunoelectrophoresis.