Immunostimulatory activity of ethanolic leaf extract of Ocimum gratissimum was investigated in albino rats using immunologic/haematologic indices. The rats were dosed orally with standard inoculum of Escherchia coli (NCIB 86) of 1x107 cfu mL-1. The extent of infection was carried out by checking the haematologic indices before, during and after treating the infection with ethanolic extract of Ocimum gratissimum. The animals were divided into four groups. The first group was dosed with 8ml of the standard inoculum for two days. The second group was dosed with the standard inoculum and treated with 250 mg mL-1 of Ocimum gratissimum ethanolic leaf extract. The third group was dosed with the extract alone while the fourth group was given normal saline and this serve as the control. The infected rat that was not given the extract showed a White Blood Count (WBC) of 4,800 mm3 before infection and increased to 13,800 mm3 during infection and later decreased to 2,400 mm3 after oral administration of the extract. The Packed Cell Volume (PCV) was 57% before infection, 47% during infection and 35% after treatment. The neutrophil and lymphocyte percentage in the differential count were 48 and 51% before infection, 62 and 37% during infection and 74 and 26% after treatment of infection respectively. For the rats treated with extract, it showed a White Blood Count (WBC) of 5,000 mm3 before infection, which decreased to 3,000 mm3 during infection and 1,700 mm3 after infections. It has a Packed Cell Volume (PCV), neutrophil and lymphocyte value of 55, 47 and 52% before infection, 50, 42 and 58% during infection and 33, 44, 56% after infection. The rats given the extract of Ocimum gratissimum alone showed a value of 4,400 mm3, 48, 41 and 58% for the WBC, PCV, neutrophil and lymphocyte before infection, a value of 3,200 mm3, 63, 43 and 57% during infection and a value of 2,100 mm3, 25, 42 and 56%, respectively after infection. The control showed only a significant increase in WBC with a value of 4,000 mm3 before infection, to 6,100 mm3 after infection and back to 4,400 mm3 after infection. The urinalysis showed a pH value of 5, negative for glucose, Ascorbic acid, Ketone, Nitrite, Protein, Bilirubin, normal for urobilinogen and negative blood value for all the groups before infection. The infected rat without administration of extract showed a pH of 7 and became positive for ketone, nitrite, protein and bilirubin urobilinogen and blood value of Ca. 250 during infection while others remain the same. After infection, the pH turned to 6, became negative for other parameters except protein and bilirubin while the treated rats remain negative. The ethanolic leaf extract of Ocimum gratissimum was found effective in inhibiting/preventing the disease condition after infection and capable of reducing excessive breakdown of red blood cells and neutralizing toxin produced by the organism.