Two vaccine and three virulent strains of infectious bronchitis virus (IBV) were used to infect day-old specific-pathogen-free chickens. Precocious development of oviducts was induced in young female chicks by oestrogen injections. Tracheal and oviduct organ cultures prepared from immunised chickens were challenged in vitro with homologous and heterologous viruses to assess tracheal and oviduct cross-protection. Tracheal cross-protection was seen between serologically related and unrelated viruses. Protection at at the level of the oviducts was lower compared to that seen at the tracheal level. However, the yields of the homologous challenge viruses from immunised oviducts were significantly reduced as compared to yields from normal oviducts. After challenge, tracheal and oviduct ciliary scores were used to group the five strains of IBV into protectotypes. The five viruses could be grouped into three protectotypes and four serotypes. The virulent IBV strains were found to have caused complete stasis of oviduct cilia when examined at three weeks post-infection. Oestrogen treatment of chicks did not significantly alter the lymphocyte proliferation responses to concanavalin A or pokeweed mitogen. The applicability of this model for the study of protection afforded to the oviducts, in terms of protection against virus-induced damage, rather than drop in egg production, is discussed.