A quantitative, semiautomated, enzyme-linked immunosorbent assay for viral antibodies was developed. The quantitation was based on the comparison of the antibody activity of a serum to a standard curve obtained from a reference serum. This comparison was made automatically by a programmable calculator connected with the spectrophotometer. The assay was simplified further by use of specially designed disposable microcuvettes and micropipettes. The reproducibility of the test was good (1 SD = 5%), and the sensitivity for antibodies to influenza A virus and respiratory syncytial virus was 100-200 times greater than that of complement-fixation or hemagglutination-inhibition techniques. The technique seems especially suitable for virus laboratories in hospitals, where the time required for specific diagnosis of viral diseases can be shortened considerably, not only because of the simple performance of the test, but also because the greater sensitivity allows the shortening of the interval between successive samples of serum down to two or three days.