Magnetic microspheres have many bio-medical applications, but they have contact with the blood before reaching to their target place in many cases, so the study of the effect of these microspheres on the blood can lead to optimization of the use of these microspheres. Five groups of normal blood samples were incubated with 10 mg magnetic chitosan microspheres for 1, 3, 6, 12, and 24 hours (h) respectively, then each group was compared to control group samples. The results showed that increase in hemoglobin conductivity with the time of incubation, no change in red blood cells osmotic fragility, no change in hemoglobin absorbance spectrum. Decrease in blood pH at 24 h, no change in neither hemoglobin electrophoretic patterns or in the blood components ultra structures. These results indicated no obvious toxicity of the magnetic microspheres on blood for incubation time up to 24 h, but on the other hand there are some changes especially at 24 h of incubation, so it is safer to avoid the remaining of the microspheres inside the body for more than 24 h.