There are conflicting reports on the appearance of blood and blood clot as seen in the human body by MRI. This study was designed to show the in vitro MR signal intensity of human blood products in the fresh state and to evaluate the serial MRI changes that occur over time (2 weeks). T1 relaxation times were also measured. Anticoagulated whole blood, plasma, serum, white blood cell concentrates, platelet concentrates, lysed red cells, red cell concentrates, and blood clot were studied. The results show that plasma and serum have similar T1 values, as do lysed and intact erythrocytes. T1 of serum and plasma rose initially and then fell with the aging of the samples. T1 of red blood cells, clot, and packed red blood cells decreased for the first 48 hr and then remained constant for 7 days before increasing to the initial values by 2 weeks. Platelets and white blood cells had little influence on the MR image. However, temperature had a significant effect on T1 and signal intensity. In vivo clots are complex mixtures of whole clot, lysing clot, serum, and plasma influenced in various ways by the adjacent normal or diseased tissues. The chemical and physical properties of the mixture change constantly. Because of the clot's complex nature, determining the age of a hematoma from the appearance of clots on the MR image may not be possible.