Peripheral blood mononuclear cells (PBMC) and polymorphonuclear leukocytes (PMNL) of patients with Argentine hemorrhagic fever (AHF) were tested as effectors (E) of antibody-dependent cell cytotoxicity (ADCC). 51Cr labeled chicken red blood cells (CRBC) coated with anti-CRBC or normal rabbit serum were used as targets (T). Three ADCC assays were performed with both effectors from patients: on admission (I), 4 days after the transfusion of immune plasma (II), and 30 days after the clinical onset (III). The ADCC values obtained displayed high variation between individuals. From the linear portions in the curves representing specific 51Cr release vs. E:T ratio plots, extrapolations were made to determine lytic units (LU), defined here as effector concentrations required to lyse 50% of the targets. The results were expressed as LU in 10(6) effector cells. The killing activity ranges of patients' PMNL (I = 1.04 +/- 0.34; II = 2.22 +/- 0.66; and III = 2.08 +/- 1.18) were not significantly different from that of 21 normal controls (1.19 +/- 0.36), except for range II (P less than 0.01). ADCC activity ranges of patients' PBMC (I = 3.40 +/- 1.06; II = 3.16 +/- 1.60; and III = 1.93 +/- 0.42) were not significantly different from that determined in 12 healthy subjects (1.86 +/- 0.40). These results demonstrate that patients' PBMC and PMNL can perform ADCC with efficiency comparable to normal effector cells, during the acute period of AHF, and in early convalescence. Consequently, ADCC can be a relevant mechanism in the clearance of Junin virus-infected cells.