Purified DNA from 42 isolates of Suid herpesvirus 1 (SHV-1) collected during 1985 from clinical outbreaks of Aujezsky's disease on Danish farms was compared by restriction fragment pattern (RFP) analysis. The BamHI generated RFPs were found to be distinguishable, thus confirming RFP analysis as a potential epidemiological tool. However, because of variations within labile genome regions of epidemiologically closely related isolates, it was concluded that comparisons of RFPs should be interpreted with great care and only on the basis of a comprehensive reference material. With this proviso, the changes in RFPs occurring during the spread of the infection might be useful markers in the evaluation of epidemiological relationships. A simple procedure for obtaining satisfactory amounts of purified SHV-1 DNA is described.