Proton nuclear magnetic resonance longitudinal-relaxation-rate measurements have been used to investigate the molecular events that occur during the early stages of the polymerization process of sickle hemoglobin. The longitudinal relaxation rates (T1-1) of the C2 protons of 11 observable surface histidyl residues in normal human adult and sickle hemoglobin in the deoxy state were measured in 0.1 M bis[(2-hydroxyethyl)imino]tris(hydroxymethyl)methane (pH 6.8) in 2H2O. These proton resonances in hemoglobin occur at a position 1.5-5.0 ppm downfield from that of residual water in 2H2O. The T1-1 values for the C2 protons of several surface histidyl residues in sickle hemoglobin in the deoxy state were sensitive to the temperature and the concentration of hemoglobin, factors known to have a profound effect on the polymerization process of sickle hemoglobin. For hemoglobin concentrations of 13.5% or less and temperatures of 25 degrees C or less, the T1-1 values in sickle hemoglobin solutions were the same as the corresponding values in normal hemoglobin, except for the C2 proton of beta 2 histidine, which had a larger T1-1 value. When the temperature or the hemoglobin concentration was increased (i) several additional histidine resonances in sickle hemoglobin solutions had larger T1-1 values than the corresponding ones in normal hemoglobin and (ii) the differences between the T1-1 values (sickle versus normal hemoglobin) of these histidine resonances as well as that of the beta 2 histidine resonance gradually increased. It is proposed that these results reflect the formation of small aggregates in the deoxygenated sickle hemoglobin solutions before gelation. In this model, the histidyl residues for which the T1-1 values are greatly increased in sickle hemoglobin solutions as compared with those in normal hemoglobin are viewed as being located in or near the "contact" areas between sickle hemoglobin molecules within the pregelation aggregates. Thus, this magnetic resonance technique can also be used to identify the intermolecular contacts in the polymerization of sickle hemoglobin.