Human apohemoglobin (globin) was spin-labeled at the beta-93 sulfhydryl groups with 2,2,5,5-tetramethyl-3-aminopyrrolidine-I-oxyl. Spin-labeled globin exhibited an EPR spectra that is less immobilized than that of spin-labeled hemoglobin, indicating the conformational difference in the vicinity of the label between hemoglobin and globin. Spectrophotometric titration of spin-labeled globin with protohemin showed that 1 mol of globin (on the tetramer basis) combines with 4 mol of hemin, producing a holomethemoglobin spectrophotometrically indistinguishable from native methemoglobin. The EPR spectrum was also changed strikingly upon the addition of protohemin. This change, however, was not proportional to the amount of hemin added, but marked changes occurred after 3 to 4 mol of hemin were mixed with 1 mol of spin-labeled globin. The EPR spectrum of spin-labeled hemoglobin thus prepared was identical with that prepared by direct spin labeling to methemoglobin. These results suggest the preferential binding of hemin to alpha-globin chains in the course of heme binding by globin. This assumption was further confirmed by preparing spin-labeled semihemoglobin in which only one kind of chain contained hemin (alpha h betaO SL and alpha O beta h SL). The EPR spectrum of the alpha h beta O SL molecule showed a slightly immobilized EPR spectrum, similar to that of spin-labeled globin mixed with 50% of the stoichiometric amount of hemin. On the other hand, the alpha O beta h SL molecule showed a distinctly different EPR signal from that of globin half-saturated with hemin, and showed an intermediate spectrum between those of beta h SL and alpha h beta h SL. These results indicate that heme binding to globin chains brings about a major conformational change in the protein moiety and that chain-chain association plays a secondary role. We conclude that hemin binds preferentially to alpha-globin chains and that the conformation of globin changes rapidly to that of methemoglobin after all four hemes are attached to globin heme pockets.