DNAs prepared from whole feline embryo cells infected with a standard laboratory strain of FHV-1, B927, and 50 wild type isolates were digested with a variety of restriction endonucleases. The resulting fragments were separated on agarose gels and Southern blotting performed. To visualize the fragments, B927 DNA was purified by pulsed field gel electrophoresis, labelled with alpha 32P and used as a hybridization probe. With most enzymes a large number of the isolates displayed altered mobilities of several fragments with some fuzzy band heterogeneity. A few isolates gave distinct cleavage patterns, in particular using the enzymes Bam HI, Cla I and Sac I. It is suggested that different strains of FHV-1 exist but that changes in the viral genome occur only sporadically and thus may not be readily detected.