Feline herpesvirus type 1 (FHV-1) isolated from cats suspected for feline viral rhinotracheitis could be divided into 3 genotypes (C7301, F2 and C7805 types) by digestion pattern with a restriction enzyme M1uI[16]. Variation of immunogenic products of 77 isolates were examined by immunoblot analysis using a polyvalent cat serum against C7301 strain, Japanese prototype strain of FHV-1. The result showed that a 36 kDa band was detected in 31 out of the 63 isolates belonging to C7301 type. However the band was not observed in the remaining 32 isolates of C7301 type and 14 isolates belonging to either F2 or C7805 type. In one isolate (91-58) of C7301 type, approximately 75 kDa band instead of 70 kDa band was observed, although the other bands were commonly observed in the remaining 76 isolates. From the results of the present and previous studies, FHV-1 field isolates could be divided into at least 4 groups and this grouping might be applicable for epidemiological studies of FHV-1 infection in the field.