Low-frequency resonance Raman spectra of transient hemoglobin species were observed within 10 nanoseconds of photolysis. The Raman frequencies of the iron-proximal histidine stretching mode for transient species having either the R or the T quaternary structure are higher than in the corresponding deoxy species. The observed frequency difference in the iron-histidine mode between the R- and T- state transients indicates that there are quaternary structure-dependent protein forces on the iron-histidine bond in the liganded hemoglobins. These differences are interpreted in terms of changes in the tilt of the histidine with respect to the heme plane.