Marek's disease virus (MDV) gene clones, RA2 and GA8, constructed in E. coli bacteriophage lambda-gt11 (gt11) were identified by a monoclonal antibody (MAb), H19.47, against a putative transformation-related viral antigen consisting of a complex of three phosphorylated polypeptides, pp41, pp38, and pp24. Both recombinants have a MDV-DNA insert of about 0.5 kb and are mapped to the region of BamHI-H or EcoRI-X fragments of the MDV genome by Southern blot hybridization. Immunoblot and immunoprecipitation with H19.47 identified a recombinant beta-galactosidase-MDV 140-kD fusion protein for RA2 and a 127-kD fusion protein for GA8. Immunoprecipitation of 35S-methionine-labeled, MDV-infected chicken embryo fibroblasts (CEF) with antisera against RA2 and GA8 fusion proteins recognized five polypeptides, of which three (p41, p38, and p24) are specified by H19.47 and the remaining two, p135 and p20, have not been previously identified. Immunoprecipitation of 32P-phosphate-labeled or 3H-glucosamine-labeled, GA-MDV-infected CEF with the antiserum against RA2 fusion protein identified a phosphorylated polypeptide of 38 kD and two glycoproteins of 60 and 49 kD, respectively. The antisera against recombinant fusion proteins thus revealed the existence of epitopes common to the phosphorylated polypeptides and other MDV-specific polypeptides. Sera from chickens or mice hyperimmunized with the purified fusion proteins reacted with serotype 1, MDV-infected CEF in the fluorescent antibody (FA) test to significant titers. These immune sera did not react with either serotype II or III, indicating the serotype specificity of the phosphorylated polypeptides.