Lactoferrin (LF) has been implicated in normal regulation of myeloid blood cell production in vitro and in vivo and abnormalities in LF-cell interactions have been associated with progression of leukemia and other hematopoietic disorders. LF may be clinically useful and for this reason we studied selected biochemical characteristics of LF. Purified human milk LF was saturated with iron from solution and analyzed by gel electrophoresis, ion-exchange and gel filtration chromatography. The metalloprotein was found to contain several molecular weight species on polyacrylamide gels. High resolution ion-exchange chromatography demonstrated the binding of LF to both anionic and cationic media under identical conditions indicating a bipolar charge distribution. Gel filtration studies revealed a tetramerized form of LF, the formation and stability of which was dependent on the ionic strength of the solution.