Mycobacterial pathogens are in the main, intracellular and proliferate within immune mononuclear cells such as monocytes. In addition to surviving killing by the bactericidal mechanisms of these cells, intracellular parasites must compete for essential nutrients. One of the most important of these is ferric iron, the availability of which is severely restricted by the host. Pathogens have evolved a variety of mechanisms for obtaining iron from host proteins such as the use of siderophores. Mechanisms of iron utilization by pathogenic mycobacteria, including M. paratuberculosis, from a variety of eucaryotic iron sources were investigated by growth in supplemented Dubos medium. Laboratory M. paratuberculosis strains were capable of growth in the absence of mycobactin. Growth was not stimulated by addition of various iron sources except microperoxidase (heme) to a small extent. Other factors such as fatty acids are thought to be more essential for growth stimulation, even under conditions of iron limitation. Ovotransferrin and inorganic ferric iron were found to be strongly stimulatory for growth of avian derived M. avium.