Food components modify the risk of cancer at a large number of sites but the mechanism of action is unknown. In the present investigation, we studied the effect of the peptide lactoferricin derived from bovine milk lactoferrin on human colon cancer CaCo-2 cells. The cells were either untreated or treated with 2.0, 0.2, or 0.02 microM lactoferricin. Cell cycle kinetics were investigated with a bromodeoxyuridine DNA flow cytometric method. The results show that lactoferricin treatment slightly but significantly prolonged the S phase of the cell cycle. Lactoferricin treatment lowered the level of cyclin E1, a protein involved in the regulation of genes required for G(1)/S transition and consequently for efficient S phase progression. The slight prolongation of the S phase resulted in a reduction of cell proliferation, which became more apparent after a long treatment time.