The "in vivo" interferon (IFN) induction capacity of two Junín virus strains--the attenuated XJCl3 and the intermediate virulent MC2--was studied in the guinea pig experimental model. Three different doses of XJCl3 strain--2,000, 10,000, and 50,000 TCID50--and a single dose of 10,000 TCID50 of MC2 were assayed. Animals were bled from day 0 to day 14 postinjection (pi) XJCl3 groups showed a constant serum IFN response. MC2 infection showed that 16% of the animals failed to develop interferonemia. The IFN activity was alpha type in most cases. The IFN serum levels induced by the MC2 strain were always lower than those attained after XJCl3 infection. The response to the positive control assayed, Newcastle disease virus, was higher and earlier than that obtained for Junín virus strains. The highest IFN individual value, which induced 160 guinea pig IFN U/ml, was detected at day 2 following XJCl3 infection, and corresponded to the highest XJCl3 dose assayed. Average values ranged from 23 to 65 guinea pig IFN U/ml, for XJCl3 groups and 15 guinea pig IFN U/ml for the MC2 group, measured at the day of maximal response. IFN presence was studied in homogenates from brain, spleen, and lymph nodes; it was detected in organs from guinea pigs infected with XJCl3 but not in organs from MC2 infected animals. IFN levels in sera or in organs failed to correlate with the histological findings. Demonstration of viral antigens in organs of infected animals, and seroconversion of Junín virus (JV) confirmed the evolution of the disease. A significant weight loss was observed just after serum IFN disappearance.(ABSTRACT TRUNCATED AT 250 WORDS)