Reoviruses contain a genome composed of 10 double-stranded RNA gene segments. A reovirus reassortant, 8B, derived from type 1 Lang (T1L) and type 3 Dearing (T3D), displayed a phenotype unlike that of either of its parents in that it efficiently induced numerous macroscopic external cardiac lesions in neonatal mice (B. Sherry, F. J. Schoen, E. Wenske, and B. N. Fields, J. Virol. 63:4840-4849, 1989). A panel of T1L/T3D reassortants and a panel of reassortants derived from 8B were used to determine whether novel T1L/T3D gene associations in 8B were responsible for its myocarditic phenotype. The results eliminated the possibility that any T1L/T3D gene combination found in 8B, from 2 genes to all 10 genes, was the explanation for its induction of cardiac lesions. This suggested that a mutation(s) in an 8B gene(s) might be responsible for induction of the myocarditis. Statistical analysis of experiments with 31 reassortants derived from 8B revealed a highly significant association (P = 0.002) of the 8B M1 gene with induction of cardiac lesions. The reovirus M1 gene encodes a viral core protein of unknown function, although evidence suggests a potential role in core structure and/or viral RNA synthesis. This represents the first report of the association of a viral gene with induction of myocarditis.