A process of replicating a virus selected from the group consisting of adenoviruses, hepadnaviruses, herpes viruses, orthomyxoviruses, papovaviruses, paramyxoviruses, picornaviruses, poxviruses, reoviruses and retroviruses, in avian embryonic derived stem cells, wherein said cells are established by a process comprising the following steps: A) isolation, culture and expansion of avian embryonic stem cells in a complete culture medium containing all the factors allowing their growth and in presence of an inactivated feeder layer of mouse fibroblasts and supplemented with animal serum; B) passage by modifying the culture medium so as to obtain progressive or total withdrawal of said factors, of said serum and of said feeder layer; C) establishing adherent or non adherent avian cell lines capable of proliferating in a basal medium in the absence of exogenous growth factors, inactivated feeder layer and a 2% or less level of serum or no serum, said process of replicating comprising the steps of: a) proliferating said cells obtained in step C) in a cultivation vessel, in suspension, in a serum-free medium N° 1, wherein said cells are allowed to aggregate to each other to form cell clumps during at least said proliferation step, and wherein said cell clumps are not disrupted by centrifugation and said cells are passaged by dilution to a larger cultivation vessel; b) infecting said cells in cell clumps with the selected virus when the cell density is of at least 1.5 million cells/ml; c) shortly before infection, simultaneously to infection, or shortly after infection adding to the cell culture serum-free medium N° 2; and d) further culturing said infected cells in order to allow virus replication; and e) optionally, harvesting said virus. A) isolation, culture and expansion of avian embryonic stem cells in a complete culture medium containing all the factors allowing their growth and in presence of an inactivated feeder layer of mouse fibroblasts and supplemented with animal serum; B) passage by modifying the culture medium so as to obtain progressive or total withdrawal of said factors, of said serum and of said feeder layer; C) establishing adherent or non adherent avian cell lines capable of proliferating in a basal medium in the absence of exogenous growth factors, inactivated feeder layer and a 2% or less level of serum or no serum, said process of replicating comprising the steps of: a) proliferating said cells obtained in step C) in a cultivation vessel, in suspension, in a serum-free medium N° 1, wherein said cells are allowed to aggregate to each other to form cell clumps during at least said proliferation step, and wherein said cell clumps are not disrupted by centrifugation and said cells are passaged by dilution to a larger cultivation vessel; b) infecting said cells in cell clumps with the selected virus when the cell density is of at least 1.5 million cells/ml; c) shortly before infection, simultaneously to infection, or shortly after infection adding to the cell culture serum-free medium N° 2; and d) further culturing said infected cells in order to allow virus replication; and e) optionally, harvesting said virus. a) proliferating said cells obtained in step C) in a cultivation vessel, in suspension, in a serum-free medium N° 1, wherein said cells are allowed to aggregate to each other to form cell clumps during at least said proliferation step, and wherein said cell clumps are not disrupted by centrifugation and said cells are passaged by dilution to a larger cultivation vessel; b) infecting said cells in cell clumps with the selected virus when the cell density is of at least 1.5 million cells/ml; c) shortly before infection, simultaneously to infection, or shortly after infection adding to the cell culture serum-free medium N° 2; and d) further culturing said infected cells in order to allow virus replication; and e) optionally, harvesting said virus.