A reverse passive hemagglutination inhibition (RPHI) test was developed for detecting rotavirus antibody in patients' sera. Sheep erythrocytes coated with guinea pig antibody against Nebraska calf diarrhea virus (NCDV) were readily hemagglutinated by NCDV in a reaction called reverse passive hemagglutination (RPHA). Inhibition of this RPHA reaction was used to detect the presence of rotavirus antibody in patients' sera which cross-reacted with NCDV. The sensitivity of RPHI was consistently at least 10 times greater than that of the complement fixation test for detection of rotavirus antibody in patients' sera. Since immunoglobulin M (IgM) antibody activity detected by RPHI was destroyed by pretreatment of serum with dithiothreitol (DDT), whereas DTT-resistant antibodies such as IgG remained unchanged, the titer of IgM antibody could be distinguished from that of DTT-resistant antibody by comparing the results of two RPHI tests performed with and without DTT pretreatment. From the IgM antibody response pattern, primary and secondary rotavirus infections could be distinguished.