Cell-mediated immune responses against Marek's disease virus (MDV) antigens were examined using reticuloendotheliosis virus (REV)-transformed cell lines of two haplo-types (B 1 9 B 1 9 and B 1 3 B 1 3 ). These cell lines were stably transfected with cloned fragments of MDV DNA resulting in the expression of the MDV-specific phosphoprotein pp38. Effector cells were obtained from P2a (B 1 9 B 1 9 ) and S13 (B 1 3 B 1 3 ) chickens at 7 days post inoculation with REV, oncogenic or attenuated serotype 1 MDV (JM-16/O and JM-16/A, respectively), serotype 2 MDV (SB-1), or herpesvirus of turkeys (HVT). Transfection of MDV genes did not influence the expression of Class I major histocompatibility complex antigens. The optimal effector to target cell ratio was determined to be 100:1. REV-sensitized effector cells lysed REV cell lines and REV cell lines transfected with MDV DNA in a syngeneic fashion. Effector cells from chickens inoculated with JM-16/O, JM-16/A, SB-1 or HVT lysed only the syngeneic, transfected cell lines, but not the parent REV cell lines. The percentage specific release caused by the MDV-sensitized effector cells was low, but statistically significant.