Potent antiviral activity against measles, influenza and polio viruses was observed in the hemolymph of Lonomia obliqua. The antiviral protein responsible for this activity was isolated, purified by gel filtration chromatography using a gel filtration column system (Superdex 75) and further fractionated using a Resource-Q ion exchange column system. Experiments with the purified protein led to a 157-fold reduction (from 3.3+/-1.25 x 10(7) to 2.1+/-1.5 x 10(5) TCID(50)mL(-1)) in measles virus production and a 61-fold reduction (from 2.8+/-1.08 x 10(9) to 4.58+/-1.42 x 10(7)mL(-1)) in polio virus production. Heating and freezing seem to have no influence over its antiviral activity. Also, the protein does not display virucidal activity and does not act on receptors on the cell membrane. The observations suggest an intracellular mechanism of action and that the protein may act as a constitutive agent that affects the innate antiviral immune response.