Lactoferrin (Lf) is an iron-binding protein which plays an important role in the host defense systems of different mucosal surfaces including the intestinal mucosa. In the present research the role of apo-Lf and iron-saturated Lf in the invasion process of enteroinvasive bacteria, grown in iron stress or excess, was investigated. As enteroinvasive bacterium, Eschericha coli HB 101 strain harboring a plasmid which contains the chromosomal inv gene from Yersinia pseudotuberculosis was utilized. The product of this gene (invasin) enables this microorganism to invade human epithelial cultured cells (HeLa). The results obtained showed that apo-Lf and iron-saturated Lf added at physiological concentration during the infection exerted a significant inhibition of adhesion (3.2 โข 105 instead 3.4 x 106 adherent bacteria grown in iron excess; 1.6x 103 instead of 2.3 x 104 adherent bacteria grown in iron-limited medium) and internalization (4.0 x 105 instead of 3.7โข 10 6 internalized bacteria grown in iron excess; 2.1 x 103 instead 2.8 โข 10 4 internalized bacteria grown in iron-limited medium). It has also been demonstrated that in these experimental conditions Lf binds to HeLa cell membrane as well as to bacterial outer membrane. It is likely that this binding interfere with the early events of interaction between bacteria and eukaryotic cells. This inhibiting effect of Lf on the invasion efficiency of E. coli HB101(pRI203) could be related to the cationic nature of the molecule, although other mechanisms cannot be ruled out.