The purpose of this study was to improve in vitro procedures for detecting cellular resistance to the avian leukosis-sarcoma group of viruses. Four feather pulp organ cultures (FPOC) were prepared from each chicken by placing pulp squeezed from feathers in wells of microtitre plates that contained culture medium. Two of the four FPOC were inoculated with Rous sarcoma virus (RSV) of subgroup A and 5 to 6 days later the fluids from all four cultures were assayed for virus by inoculating chicken embryo fibroblasts (CEF) and examining for development of foci of transformed cells. Prior to the second assay of culture fluids, quail cells transformed by envelope-defective RSV [R(-)Q cells] were added to some RSV-inoculated and uninoculated FPOC. The R(-)Q cells produce infectious RSV when infected with avian leukosis virus (ALV), and hence made it possible to detect ALV in FPOC. Status of host infection was also assessed by tests for virus neutralising antibody and the enzyme-linked immunosorbent assay for group specific viral antigen. In one experiment FPOC from chickens not exposed to ALV were susceptible to RSV throughout the 140-day test period. In contrast, FPOC from ALV-inoculated chickens were usually infected with ALV and were resistant to RSV. FPOC from chickens reared in contact with the inoculated group for 121 days were free of ALV and were unexpectedly resistant to RSV. Two other experiments supported the observation that genetically susceptible chickens acquire cellular resistance to RSV as a result of persistent or transient ALV-infection. In Cornell K strain chickens there was close agreement between cellular susceptibility based on tests on FPOC prepared prior to inoculation of chickens with ALV and for antibody following inoculation with ALV. A New Hampshire strain showed a high degree of genetic cellular resistance by these test procedures.