A specific antiserum was raised against the pestivirus inducing hog cholera (hog cholera virus, HCV). Using immunoprecipitation and SDS-PAGE, this antiserum served for comparison of HCV-induced proteins with those from a related and better characterized pestivirus, bovine viral diarrhea virus (BVDV). In addition to immunological relationships, the apparent molecular weights of some proteins induced by both viruses were quite similar. HCV genomic RNA was found to be about 12 kb in length, comparable to BVDV RNA. cDNA was synthesized starting from RNA isolated from partially purified virions and cloned in lambda gt11. Screening with the antiserum resulted in identification of several positive clones. Partial sequencing of one HCV-derived cDNA clone revealed a high degree of homology to a portion of the BVDV sequence.