Parthenium hysterophorus has widely reported traditional uses which include its use for stimulation and purification of platelets as well as body defense. Experiments were carried out to assess the impact of Parthenium hysterophorus leaf extract on hematological parameters of wistar rats. The results revealed that the extract (10–100 mg/kg p.o.) produced no significant change in the packed cell volume, haemoglobin, red blood cell count, total leucocyte count, monocyte, basophil, eosinophil, bleeding time and clotting time. The dose of 100 mg/kg p.o. produced a significant (P<0.05) increase in neutrophil and decrease in lymphocyte. Platelet count was significantly (P<0.05) increased at the dose of 100 mg/kg p.o. The results did not support the traditional use of the plant leaf for stimulation of blood production. It however showed improvement of non-specific immune responses involving phagocytosis and inflammation.