Persistent infection of C3H/St mice with lymphocytic choriomeningitis virus (LCMV) strain Armstrong leads to disordered growth and hypoglycemia. Both host and viral determinants contribute to this growth hormone (GH) deficiency syndrome (GHDS). Development of the GHDS correlates with the virus's ability to replicate in the GH-producing cells and cause reduced levels of GH synthesis. LCMV strain WE infects few GH-producing cells and does not cause GHDS in C3H/St mice. We show here that clonal variants isolated from the GHDS-nil WE population are able to replicate at high levels in GH-producing cells and cause GHDS in C3H/St mice. These variants are stably maintained, but phenotypically silent, within the GHDS-nil WE population.