In conjunction with field trials for a vaccine against Argentine Hemorrhagic Fever (AHF), small mammals were trapped during a 28-month period (1 November 1987 to 13 March 1990) in 3 epidemiologically defined areas of the central Argentine pampas: northern and central Buenos Aires provinces were included in the AHF "historic" area, where the disease was common 15-20 years ago, but case rates are currently low; southern Santa Fe province is the current high-incidence area for AHF; the nonendemic area was represented by two localities 60-90 km beyond the northernmost extension of human disease. Animals were live-trapped for 3 days per month in permanent "mark-recapture" grids in each of the 3 areas. Samples of blood, sera, and oral swabs were taken from these animals before they were marked and released at the site of capture. In addition, "removal" traplines provided animals from 16 localities in these 3 areas which were sacrificed to obtain samples of organs in addition to the aforementioned samples. Samples were tested for the presence of Junin virus (JV) antigen by enzyme immunoassay (ELISA). In this assay, a pool of 13 mouse anti-JV glycoprotein and nucleocapsid monoclonal antibodies adsorbed to the surface of microtiter plates was used to capture JV antigen in sample suspensions. A polyclonal rabbit anti-JV antiserum was added as a detector antibody, and an anti-rabbit antibody conjugated to horseradish peroxidase applied with substrate to complete the sandwich.(ABSTRACT TRUNCATED AT 250 WORDS)