AbstractOlfactory ensheathing glia (OEG) have been used to improve outcome after experimental spinal cord injury and are being trialed clinically. Their rapid proliferation in vitro is essential to optimize clinical application, with neuregulins (NRG) being potential mitogens. We examined the effects of NRG‐1β, NRG‐2α, and NRG3 on proliferation of p75‐immunopurified adult OEG. OEG were grown in serum‐containing medium with added bovine pituitary extract and forskolin (added mitogens) or in serum‐containing medium (no added mitogens). Cultures were switched to chemically defined medium (no added mitogens or serum), NRG added and OEG proliferation assayed using BrdU. OEG grown initially with added mitogens were not responsive to added NRGs and pre‐exposure to forskolin and pituitary extract increased basal proliferation rates so that OEG no longer responded to added NRG. However, NRG promoted proliferation but only if cells were initially grown in mitogen‐free medium. Primary OEG express ErbB2, ErbB3, and small levels of ErbB4 receptors; functional blocking indicates that ErbB2 and ErbB3 are the main NRG receptors utilized in the presence of NRG‐1β. The long‐term stimulation of OEG proliferation by initial culture conditions raises the possibility of manipulating OEG before therapeutic transplantation. © 2007 Wiley‐Liss, Inc.