An indirect enzyme-linked immunosorbent assay was developed for rapid detection of serum antibodies against the virus responsible for the porcine reproductive and respiratory syndrome (PRRS). This test is more sensitive than the reference technique (immunoperoxidase test applied to cultures of alveolar macrophages), particularly for detecting animals at the stage of seroconversion. It is also very specific for PRRS virus, because all specific hyperimmune sera against other porcine viruses, and all serum samples taken from herds before the disease appeared in western France were negative. The test has been used for routine diagnosis of PRRS. The results obtained during nine months from over 21,000 samples have confirmed the value of the test for diagnosis and epidemiological surveillance of the disease.