The role that polymorphonuclear leukocytes (PMN) may play in Argentine hemorrhagic fever (AHF), an endemo-epidemic disease caused by Junín virus (JV), was investigated in experimentally infected guinea pigs depleted of PMN by means of specific antiserum. In leucopenic animals the evolution of the infection with a highly pathogenic strain of JV was more severe, with earlier mortality and higher virus yields in blood and viscera. The pathological study showed similar lesions in both the control and PMN-depleted animals with the exception of the lung, which showed the pathological picture of the human "pulmonary distress syndrome of the adult" in nontreated guinea pigs and appeared histologically unaltered in the PMN-depleted animals. On the basis of these results it is suggested that in AHF, PMN play a dual role. In the first stage of infection they display a defensive antiviral action, but later on they participate in the pathogenesis of tissue damage.