The complete amino acid sequence of the Puumala (PUU) virus nucleocapsid protein (N), deduced from the genome of the prototype strain Sotkamo, was synthesized as decapeptides with 5-amino-acid overlaps. By use of the PEPSCAN method, 86 peptides were examined for reactivity with sera from serologically confirmed nephropathia epidemica (NE) patients and 11 PUU virus N-specific bank vole monoclonal antibodies. The human sera showed reactivity with several different regions, while only one of the monoclonal antibodies reacted with one single peptide. Sequences were selected by this PEPSCAN analysis of human antibody reactivities, and five 15-amino-acid peptides were synthesized and evaluated as antigens by an enzyme-linked immunosorbent assay (ELISA). Peptide-reactive antibodies of the immunoglobulin M (IgM) class were measured in serum samples drawn from patients with acute NE. In comparison with the results of a mu-capture IgM ELISA using native PUU virus antigen, only a few serum samples were found positive (sensitivity, 2 to 10%). Interestingly, when antibodies of the IgG class were measured, the sensitivities of the five peptide ELISAs were found to be 79, 46, 2, 100, and 40%, respectively, as compared with the sensitivity of an IgG ELISA based on native viral antigen. The IgG reactivities of sequentially drawn sera from NE patients with the two peptides giving the highest assay sensitivities were analyzed and compared with their reactivities with native viral antigen. All patients had detectable anti-peptide IgG in the acute-phase sample, which, however, had totally declined in samples drawn after 2 years. The opposite pattern was seen with native viral antigen, in which case all patients showed the highest levels of specific IgG after 2 years. The results suggest the presence of epitopes specific for the acute IgG response.