An enzyme-linked immunosorbent assay (ELISA) was developed for the detection of antibodies to influenza A/Hong Kong/1/68, influenza A/Victoria/3/75, influenza B/Hong Kong/5/72, and parainfluenza type 1 viruses. Development and standardization of the method indicated that an acetone-ethyl alcohol mixture was a suitable fixative for the preparation of the solid-phase coupled antigen. The addition of sodium azide to the enzyme-conjugated solution and the concentrations of the enzyme-conjugate antiglobulin and test sera employed were all critical factors in the success of the ELISA procedure. The ELISA test was specific; there was no cross-reaction between influenza A and B or parainfluenza type 1 viruses. The concordance between ELISA and hemagglutination inhibition results suggested that both tests probably detected the same type of antibodies. The ELISA procedure was 8 to 64 times more sensitive than complement fixation and/or hemagglutination inhibition tests. Low levels of antibody in patients' sera were detected only by the ELISA test. During the course of the testing period false positive reactions were not encountered. The results of ELISA could be obtained within 3 h. The ELISA test required a very small amount of serum and, therefore, offered an opportunity to detect the presence of maternal antibodies to influenza viruses in blood collected from infants by heel prick.