The in-vivo antimalarial activity of the aqueous leaf extract of Phyllanthus amarus against quinine-sensitive strain of Plasmodium berghei-infected mice was investigated. The twenty-two (22) albino mice used were divided into six groups. Three groups were treated with different doses of Phyllanthus amarus extracts (100, 250 and 500mg/kg), another group was treated with the standard drug quinine (10mg/kg), the other groups were not treated. The extract, at all concentrations, produced considerable antiplasmodial activity by reducing the level of parasitaemia which was comparable to that the standard drug quinine, in a 4 day suppressive test. In an 8th day suppressive test, the 250mg/kg leaf extract had the highest curative activity (85%) as comparable to the standard drug quinine (81%). The mean survival time ranged from 11 to 15 days. The antimalarial effect of the aqueous extract of Phyllanthus amarus and its influence on hematological profiles in treated and untreated mice were assessed. The untreated group of mice infected with the quinine-sensitive strain of Plasmodium berghei recorded a significant (P< 0.05) reduction in PCV, RBC, MCV and neutrophils observably from day 4-8 post-infection while WBC and Platelets counts increased significantly (p<0.05) from day 4-8 post-infection. Administration of the aqueous extract of Phyllanthus amarus at different doses 100, 250, 500 mg/kg respectively to mice infected with Plasmodium berghei resulted in the normalization of hematological indices in the groups of mice infected and treated with the plant extract when compared with the data obtained for the experimental control groups of mice. Although, it was observed that the MCV and MCHC values did not vary significantly in the treated groups when compared with experiment control groups. Treatment with the 250 and 500mg/kg of the extract caused the serum AST, ALT and ALP levels of the mice to be within the normal range as comparable to mice treated with quinine. Phytoche