A recombinant virus, CVI/rpp38, was developed from the Marek's disease virus (MDV) CVI988/Rispens vaccine strain. This recombinant was obtained by transfection of CVI988/Rispens-infected chick embryo fibroblasts (CEFs) with plasmid pHA25 DNA containing pp38 gene from GA strain of MDV. Monoclonal antibody (MAb) H19 which reacts with pp38 from GA but not with that from CVI988 was used to screen for recombinant viruses in transfected cell culture plates by immunofluorescent assay (IFA). A positive plaque was isolated, propagated, and purified from cell-free virus particles after sonication of infected CEFs. The mutant CVI/rpp38 was not only reactive with MAb H19 in IFA but also in immunoprecipitation. A 38 kDa protein was immunoprecipitated from the CVI/rpp38 mutant virus but not from parental CVI988 virus. DNA sequence of the mutant virus showed a substitution of G at position 320 by a resulting in a change of an amino acid residue from arginine to glutamine. Comparison of nucleotide sequence of pp38 from strains GA, Md5 and Md11/75c/R2 and CVI988 revealed change to glutamine in this position. The result of this study provides a direct evidence for the location of the identified H19 epitope in pp38. This mutant is potentially useful to further explore the biological function of pp38 and its H19 epitope.