Several blood group systems have been discovered but amongst the most important is the ABO group. Landstainer demonstrated four groups according to antigens available A, B, AB and O and showed that an individual possess antibodies against those antigens he lacked on the red cell. Several other red cell blood groups and hence antigens have been shown but the rhesus group is another of utmost importance. Cells which have Rhesus antigen on their surface are described as Rhesus positive while those without this rhesus antigen are Rhesus negative. Osmotic fragility is a test to measure red blood cell (RBC) resistance to hemolysis when exposed to a series of increasingly dilute saline solutions. Osmotic fragility tests are based on the measure of red blood cell lysis as a function of osmotic stress. When erythrocytes are placed in hypotonic solutions, they begin to take on water osmotically. This results in swelling of the cell until the critical volume is reached, afterward the membrane at first leaks and then bursts releasing hemoglobin. This study evaluated the comparison of blood groups and osmotic fragilities under different concentrations of saline solution respectively, the blood group that has the highest frequency of haemolyses is considered most osmotically fragile. The purpose of this study is to aid in diagnosis of disease with red cell membrane abnormality by providing which blood group is more fragile osmotically and also to add to the general knowledge in medical physiology. An ethical clearance was sought from Ahmadu Bello University Teaching Hospital Ethical Committee on Human Research. Seventy blood samples from different subjects were used which includes different ethnic groups, age and sex. Using a 5ml syringe, 2ml of blood were collected and the experiment was carried out in the laboratory and both the blood group and osmotic fragility were determined. This study showed the mean value and standard deviation of 0.4694 ± 0.11909, 0.6050 ± 0.11459, 0.4917 ± 0.09003 and 0.7000 ± .14142; and percentage haemolysis of 36%, 20%, 12% and 2% for O, A, B, and AB groups respectively and the groups were compared using analysis of variance (ANOVA) followed by Turkey’s post hoc test. The result in comparison shows that blood group O is more fragile osmotically followed by B, A then AB is least fragile which was statistically significant at p value of 0.000. Therefore, this result will help providing information in medical practitioners in blood maintenance and diagnosis of patients with abnormal red cell membrane.