The effect of peripheral blood lymphocytes (PBL) on cell-free Marek's disease virus (MDV) was studied in vitro in PBL from intact or embryonal bursectomized (Bx) chicks infected with the apathogenic SB-1 isolate of MDV. The PBL were harvested purified and were mixed, 200 or 2,000 to 1, with cell-free GA-5 of MDV. Virus survival was assayed after incubation for 1 hour at 36 C by the standard assay for cell-free MDV. PBL from intact but not Bx chickens were able to inactivate virus. When the effective suspension of PBL was depleted of macrophages, inactivation no longer occurred, suggesting that it resulted from cooperation between macrophages and B-cells.