Reovirus has been shown to lyse most transformed cells while establishing a persistent or abortive infection in non-transformed cells. Developing methods to identify the reovirus genes associated with oncolysis is an important step toward understanding the mechanisms involved. This report is the first to develop and apply the use of monoreassortants and reverse genetics to identify an individual reovirus gene associated with reovirus oncolysis. Infection with reovirus serotypes 1/Lang, 2/Jones or 3/Dearing of cells transformed with a normal copy of c-Ha-RAS (N1 cells) or with a normal copy of c-Myc (Myc-3 cells), produces large amounts of progeny virus of all three serotypes and results in lysis of both these cell lines. Infection of cells transformed with a mutant c-Ha-RAS gene (T1 cells) with either serotype 1/Lang and 2/Jones results in the production of large amounts of virus and lysis of the cells. In sharp contrast, serotype 3/Dearing virus infection of these cells produced small amounts of virus and resulted in limited lysis of these cells. Using monoreassortants and reverse genetics we exploited this phenotypic difference between the three serotypes to identify a single reovirus gene linked to the preferential lysis of the T1 cells, the S4 gene.