Neutrophilic polymorphonuclear leukocytes (PMN) bind specifically lactoferrin. This binding was characterized by two types of binding sites: with higher affinity and low capacity and another site with lower affinity and higher capacity (respectively Kaff 1 = 2.2 X 10(9) M-1 and about 39,000 binding sites per cell and Kaff2 = 0.6 X 10(9) M-1 and 76,000 binding sites per cell). The lactoferrin binding to PMN leucocytes depends on the concentration of labelled lactoferrin, the number of cells and the time for incubation. The presence of lactoferrin receptors on the PMN leucocytes is connected most probably with its effect on the cell function of these cells.