A sensitive, quantitative and reproducible plaque assay for the measurement of the simian rotavirus SAII is described. Plaque formation required the presence of the facilitators pancreatin or trypsin and diethylaminoethyl-dextran in the agar overlay. SAII produced plaques in three continuous primate cell lines: MA-104, CV-1 and LLC-MK2. MA-104 cells were the most sensitive.