AbstractInoculation of adolescent CD‐1 mice with one variant of coxsackievirus B3 (CVB3m) results in induction of readily observable myocardial lesions, whereas inoculation of siblings with a second variant (CVB3o) results in little or no myocarditis. These variants could not be distinguished from each other on the basis of replication properties in HeLa cells or cardiac tissues in vivo, sensitivity to human interferon in HeLa cells, induction of interferon in the mouse, generation of detectable levels of defective‐interfering particles in HeLa cells or in cardiac tissue in vivo, stimulation of serum‐neutralizing antibody titers, nor in their rate of clearance by the spleen. Infectivity of CVB3owas slightly more heat labile at 34oC than CVB3m. Little if any replication of either CVB3oor CVB3moccurred in either adherent or nonadherent populations of normal murine lymphoid cells. Cardiac tissues from mice inoculated with CVB3mbut not CVB3ocontain new antigens that can inhibit migration of sensitized lymphocytes from CVB3m‐immunized mice in an in vitro cell‐migrationinhibition assay. However, the CVB3ovariant was shown to have the genetic capability of inducing myocarditis if the mice were treated with cyclophosphamide prior to virus inoculation. These results suggest, in agreement with our previously published work, that induction of myocarditis by CVB3 requires destruction of myocytes by virus and subsequent stimulation of cell‐mediated responses to new antigens produced in the myocardium during virus