The report describes the development of an enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies against the human reovirus-like agent of infantile gastroenteritis (HRVLA). This ELISA system proved to be four times as sensitive as the standard anti-HRVLA fluorescent-antibody assay and ten times as sensitive as the standard anti-HRVLA complement fixation assay. In addition, the ELISA was capable of determining immunoglobulin G (IgG) and IgM subclasses of anti-HRVLA antibody using a single dilution os serum. With this assay, it was discovered that 11 of 21 infected children had anti-HRVLA IgM in their acute sera before the appearance of anti-HRVLA IgG. ELISA is a useful tool in the evaluation of immunological response to HRVLA infection.