Iron is an important factor for growth, virulence and immunogenicity of the species Pasteurella multocida. This has been demonstrated in numerous experiments with bacterial cultures in vitro and immunized and not immunized animals in vivo (mice, piglets, calves). Iron substrates or iron chelators affect in different manner the virulence of P. multocida in vivo, depending on chemical character of the given compounds, their dose, route and time of application, and also depending on the host. P. multocida has an up to time unknown iron transport system, which can acquire the essential iron from physiological substances, such as heme, ferritine, transferrine, lactoferrine etc. This conclusion results from in vitro experiments with growing cultures, with insertion of radioactive iron (Fe-59) from different sources, and with iron solubilization in neutral pH ranges. In the same way, the iron of iron dextran and low molecular iron compounds is available for P. multocida. Iron of unphysiological complexes, potassium ferrocyanide, and ferrocene is unavailable. On the other side such iron chelating agents as nitrilotriacetate, tirone, ferrocene, citrate, EDTA, and apotransferrine do not or only a little affect growth, and such chelators as alpha, alpha'-dipyridyle, phenanthroline and the microbial siderophores deferrioxamin B and enterobactin are inhibitory substances for multiplication of P. multocida. This substances also inhibit the insertion of Fe-59 into the bacterial cell. The conclusion is drawn that neither enterobactin nor deferrioxamine B as typical representatives of phenolate or hydroxamate siderophores take part in Fe-transport of P. multocida.