Studies on the natural history and prevention of viral hepatitis have been in progress at the Willowbrook State School since 1956 [1-5]. The background for this investigation was described in detail in a recent publication [5]. During the course of these studies, 2 types of hepatitis were identified; one was designated MS-1 and the other MS-2. The MS-1 type resembled classical viral hepatitis, type A or infectious hepatitis (IH). The MS-2 type resembled viral hepatitis, type B or serum hepatitis (SH). Availability of separate serum pools containing the MS-1 and MS-2 strains of hepatitis virus provided pedigreed materials for the study of IH and SH viruses. Previous experience revealed that both viruses were infective after exposure to a temperature of 56 C for 30 min [1, 6]. The present study was undertaken to evaluate the effect of boiling (98 C for 1 min) on the infectivity and immunogenic capacity of the MS-1 and MS-2 serum pools.