A large number of medicinal plants and their purified constituents have been shown to have beneficial therapeutic potentials. In this study, ethanolic extract of Bidens pilosa was evaluated for its invivo activity on haematological parameters in Swiss albino rats orogastrically dosed with Escherichia coli O157:H7. Fifteen Swiss albino rats were used for the study. The animals were divided into five groups of three rats each. The first, second and third group of rats were orogastically dosed with 9.1 × 104 cfu/ml of E. coli O157: H7 to induce infection. The first group was treated with 800 mg/kg Body weight (Bw) of the ethanolic extract of B. pilosa, the second group was treated with Ofloxacin (16mg/kg Bw), while the third group was not treated. The fourth group was given only the plant extract, while the fifth group was given sterile distilled water. The results of the haematological assay indicated that: the infected-untreated rats showed lowest mean values of PCV (34.00±2.50a), RBC (6.54±0.45a) and HB (11.50±0.83a); and highest ESR (4.50±0.50c). In the infected-extract-treated group, a significant increase in the PCV (45.00±1.00b) and HB (15.00±0.33b) was observed. The group fed with extract alone had the highest mean values of PCV (51.00±1.00b), RBC (11.10±0.95c) and HB (17.00±0.33b). Similar pattern was observed for the results obtained for the white blood cell differential count. The infected-extract-treated group, and the group to which only extract was administered without infection showed significant increase in lymphocyte count (61.00±1.00ab) and (73.50±2.50c) respectively. Conversely, the infected-untreated group showed a decline in lymphocyte count (54.50±3.50a). The results obtained from this study revealed that ethanolic leaf extract of Bidens pilosa exhibited haematopoietic potential and tends to modulate the values of White Blood Cell differential count in Swiss albino rats.