The stability of the infectivity of Simian rotavirus, SA11, has been analysed and compared to the stability of reovirus type 1. SA11 infectivity was stable to freeze-thawing, sonication, incubation at 25 degrees C overnight or at 37 degrees C for 1 h and to treatment with acid, ether, chloroform and Genetron. In contrast to reovirus, the infectivity of SA11 was more rapidly inactivated by heating at 50 degrees C. SA11 infectivity was inactivated above pH 10.0 and by heating at 50 degrees C in 2 M-MgCl2, but was stabilized by heating in 2 m-MgSO4; reovirus 1 infectivity was enhanced by heating in MgCl2. Both SA11 and reovirus 1 were inactivated by freezing in MgCl2. These results show that rotaviruses and reoviruses can be distinguished by their patterns of inactivation by physical and chemical agents.