This assay is a direct ELISA detecting Human IgG and/or IgM antibody specific for LASV NP. Diluted samples, Reference, and Negative Control are incubated in microwells coated with a mixture of recombinant LASV NP antigens. Incubation allows the anti-LASV antibody present in the samples to react with the immobilized antigen mixture. After the removal of unbound serum or plasma proteins by washing, anti-human IgG (or IgM) antibodies, labeled with horseradish peroxidase (HRP), are added forming complexes with the bound IgG (or IgM) anti-LASV antibody. Following another washing step, the bound enzyme-antibody conjugate is assayed by the addition of TMB substrate. Color develops in the wells at an intensity proportional to the concentration of IgG (or IgM) anti-LASV antibody in the sample. Optical Density (O.D.) results are obtained by reading the absorbance (A450nm minus A620nm) using an ELISA plate reader. It is recommended that the user establish a cut-off for the study population using LASV sero-negative samples. It is also recommended that LF IgG and IgM positive convalescent LF sample from the study population be included in each assay as an additional reference sample.