Allium cepa (onion) and Allium sativa (garlic) are bulbous herbs used as food item, spice and medicine in different parts of the world. The effects of onion and garlic on the osmotic fragility of red blood cells in albino rats were assessed in vivo and in vitro. In the in vivo studies, five albino rats weighing between 150 - 200g composed each of three study groups. Group A were administered 150mg/Kg body weight aqueous onion preparation; Group B 75mg/Kg body weight aqueous onion and 75mg/Kg body weight garlic preparations; and Group C served as the control and were administered distilled water. The treatment regimens were orally administered thrice a week, for a period of four weeks by gavages. The in vitro erythrocyte osmotic fragility was also evaluated in 12 Wistar rats that were not pre-treated with either onion alone or onion and garlic. The animals were divided into three groups. Blood samples from group A rats were treated with 150mg onion while blood from group B rats was treated with 75mg onion and 75mg garlic extracts. Group C served as the control and were treated with normal saline and osmotic fragility assays were carried out. The degree of haemolysis was greater in the treatment group compared to control and the percentage haemolysis was greater in blood samples with onion and garlic compared to the onion group. The same observation was made in the in vitro study, but the degree of haemolysis was significantly higher in in vitro than the in vivo experiments. It is concluded that onion and garlic increase the osmotic fragility of red blood cells in albino rats.