Proton spin-spin relaxation time (T2) was measured at room temperature(-21o C) at 20 MHz, in human whole blood, plasma, serum, packed red cells, and hemoglobin for normal, and whole blood and plasma for selected pathological samples. Plasma and serum T2 values are equal within experimental errors. The average plasma and packed red cells T2 for normal blood remains essentially constant at 0.38 s and 0.12 s in the normal range of hemoglobin concentrations. T2 of the whole blood, averaging (0.24 + .04) s, is linearly dependent on the hemoglobin concentration, and increases by about 33% from 9 to 17 g hemoglobin / 100ml blood. T2 of extracted oxyhemoglobin varies predictably with time from extraction. Pathological samples show noticeable differences in plasma T2 values, indicating the possibility of using NMR to distinguish between normal and pathological samples.