Digoxigenin-dUTP-labeled DNA probe was prepared from a cDNA clone complementary to the gene encoding S1 region of the spike protein of infectious bronchitis coronavirus (IBV) strain M41. The probe exclusively reacted with four strains at 56 degrees C which were grouped to the same serotype as the strain used for the probe. In contrast, at 68 degrees C, the probe reacted only with the homologous strain and did not react even with the strains belonging to the same serotype. The dot-blot hybridization thus appeared serotype-specific at 56 degrees C and strain-specific at 68 degrees C. In addition, it was revealed that the S1 gene has some nucleotide sequence variation even among strains in the same serotype. This technique should be applied to determining serotypes of the virus isolates and to differentiating field isolates from the vaccine strain.