Japanese quail cells transformed by the replication-defective, Bryan high-titer strain of Rous sarcoma virus, BH RSV(-)Q, clone 3, revealed intracytoplasmic A-type particles after dexamethasone treatment. In the absence of dexamethasone, or when superinfected with a helper virus in the presence or absence of dexamethasone, no such particles were observed. Chick embryo cells (CEC) infected with a temperature-sensitive sarcoma virus mutant, LA 334, coordinately defective for transformation and viral replication, showed abnormal accumulation of viral core substances and aberrant budding at the non-permissive temperature (41 degrees). CEC infected with LA 334 and treated with dexamethasone at 41 degrees resulted in more extensive accumulation of abnormal budding without increased release of viral particles. Dexamethasone, however, did not lead to abnormal morphogenesis of virus under permissive conditions. The selective effects of dexamethasone on the morphogenesis of these two mutants of avian sarcoma virus are discussed.