RNA was isolated from two strains of Marek's disease virus (MDV-Z and MDV-B). The virus was grown in duck embryo fibroblasts (DEF) for 96 hr, 72 hr in the presence of phosphonoacetic acid (PAA) and 24 hr in the presence of cycloheximide added at the time of infection. With the use of DNA probes representing about 80% of the MDV genome, an extensive Northern blot analysis of the RNA was carried out. A similar analysis was done with RNA extracted from the MDV-transformed cell line MSB-1. This study revealed 42, 25 and 29 discrete viral RNA transcripts in MDV-Z and MDV-B-infected DEF and in the MSB-1 cell line, respectively, ranging in size from 0.8 to 13 kb. In MDV-Z-infected DEF, there were twelve late RNA species, two early and eight immediate-early viral transcripts. In MDV-B-infected DEF there were eleven late RNA species, two early and seven immediate-early viral transcripts. The RNA species were homologous for all the probes used except the BamHI-G DNA fragment where no RNA transcripts were detected in the MSB-1 cell line. The RNA transcripts were used to produce a preliminary viral RNA map. Comparison of the location and sizes of the viral RNA transcripts in MDV-infected and MDV-transformed cells revealed several differences.