Cyclophosphamide is an alkylating agent used to treat both malignant and non-malignant immune-mediated inflammatory disorders in humans. It is also known as a potent immunosuppressive drug in humans and experimental animals. The aim of this study was to evaluate the effects of oral administration of lactoferrin (LF) on cellular responses and reconstruction of the lymphocyte pool in mice treated with cyclophosphamide (CP). Twelve week-old CBA mice were given a single intraperitoneal (i.p.) dose of CP (400 mg/kg body weight), then were treated per os with seven doses of LF (1 mg/dose) on alternate days. We demonstrated that the magnitude of delayed type hypersensitivity to ovalbumin, strongly diminished by CP action, was reconstituted by LF. Oral LF treatment also resulted in partial recovery of Concanavalin A-induced splenocyte proliferation. Blood profile analysis revealed elevation of leukocytosis by LF in CP-treated mice (from 64.9 to 84.76% of the control value). LF also caused substantial restoration of the percentage of the lymphocyte population in circulating blood (from 43.4 to 60.2% of the control values). LF alone had no effect on the neutrophil/lymphocyte ratio in normal mice, however, the total number of leukocytes decreased by 23.25%. Furthermore, we showed that LF increased the cellularity of spleens isolated from CP-treated mice (from 53.2 to 78.8%) and the content of peritoneal and alveolar macrophages (elevations from 50.6 to 67.3% and from 65.2 to 83.6%, respectively). Lastly, using panning technique, we demonstrated that LF strongly elevated the pool of CD3+ T cells in normal and CP-immunocompromised mice and CD4+ T cell content. In conclusion, we showed for the first time that lactoferrin, given orally to CP-immunosuppressed mice, could reconstitute a T-cell mediated immune response by renewal of the T cell pool.