Human heme-free hemoglobin was labeled at beta 93 with either N-iodoacetylaminoethyl-5-naphthalene-1-sulfonate (AEDANS) or fluorescein iodoacetamide (FIA), at beta 1 with pyridoxal-5-phosphate (PLP) and at the heme pocket with anilinonaphthalene-8-sulfonate (ANS). The correlation times associated with these probes ranged from approximately 12 ns for FIA and AEDANS to nearly 20 ns for ANS and PLP. This indicates the presence of internal flexibility in apohemoglobin with librational motions dominated by the mobilities of the monomeric subunits and of the entire dimeric molecules, variously weighted by the different probes. It was not possible to detect motions characterized by correlation times of about 5 ns such as were present in AEDANS-labeled oxyhemoglobin.