The aim of this study was to investigate the effects of lactoferrin (LF) on the proliferative response of human peripheral blood mononuclear cells (PBMC) induced by phytohemagglutinin (PHA) and alloantigens in a two-way mixed lymphocyte reaction (MLR). The proliferative responses were measured by MTT colorimetric assay and presented as a proliferation index (PI) or as changes in PI caused by LF relative to PHA or MLR controls. We found that the effects of LF in both experimental models were differential and dependent on an individual PBMC reactivity, mitogen or alloantigen and LF concentration. Generally, lymphocytes from donors responsive to LF exhibited higher proliferation indices to PHA when compared with non-responsive individuals. Lactoferrin at low doses showed regulatory effects, whereas at higher doses the proliferation was significantly reduced. Mixed lymphocyte reaction was generally inhibited by LF. The results suggest that the differential action of LF might be due to its ability to sense the activation status of lymphocytes.