Disclsoed is a process for obtaining continuous genetically stable avian cell lines derived from avian embryonic stem cells (ES), wherein said avian cell lines are capable of proliferating in a basal medium in the absence of exogenous growth factors, serum and/or feeder layer and do not produce replication-competent endogenous retrovirus particles, said process comprising the steps of: a) isolating bird embryo(s) at a developmental stage around oviposition, wherein the genome of said bird does not contain endogenous proviral sequences susceptible to produce replication competent endogenous retroviral particles； b) suspending avian embryonic stem (ES) cells obtained by dissociating embryo(s) of step a) in a complete culture comprising: Insulin Growth factor 1 (IGF-1) and Ciliary Neurotrophic factor (CNTF) as only growth factors； and animal serum； c) seeding the suspension of ES cells obtained in step b) on a layer of feeder cells and further culturing the ES cells for at least one passage； d) withdrawing IGF-1 and CNTF from the culture medium, and further culturing the cells for at least one passage； e) progressively decreasing the concentration of feeder cells in the culture medium so as to obtain a total withdrawal of feeder layer after several passages, and further culturing the cells； and f) obtaining adherent avian cell lines derived from ES cells capable of proliferating in a basal medium in the absence of growth factors, feeder layer, and wherein said continuous genetically stable avian cell lines do not produce replication-competent endogenous retrovirus particles.
