Serum human IgG levels were analyzed by solid phase ELISA assays using alkaline phosphatase-conjugated goat anti-human IgG antibody incubated for 1 hour at 37C. Assays was developed using 0.1% p-nitrophenyl phosphate (Sigma) as the substrate. Antibodies to the Seoul (strain 80/39) virus was detected in human sera by western blot analysis using alkaline phosphatase-conjugated goat anti-human IgG antibody at a 1:1000 dilution. Blots were developed using nitroblue tetrazolium and 5-bromo-4-chloro-3-indoyl-phosphate substrate (Sigma).