The electron spin resonance spectra of the isolated ferri α, β and γ chains of human adult and foetal hemoglobins have been obtained (at 77 °K) and compared with those of the products of their recombination, namely the respective methemoglobins. All the hemoproteins examined gave essentially similar spectra and g-values for a given liganded form (aquo, hydroxy, fluoride, azide); however, important differences between the aquo forms appeared on comparing the line-widths and signal heights per unit heme concentration. The aquo derivatives of α+SH and α+PMB†PMB, βPMB and γPMB, chains with thiol groups transformed into paramer-curibenzoate derivatives by reacting with p-hydroxy mercuribenzoate; αSH, βSH and γSH, chains with free thiol groups. αSH, αPMB, etc.: ferro chains. α+SH, α+PMB, etc.: ferri chains. Mb, myoglobin; Mb+, met- or ferrimyoglobin; HbA, adult human hemoglobin; HbFt, foetal human hemoglobin; Hb+A and Hb+ Ft, ferrihemoglobins; E.S.R., electron spin resonance.+ and γ+ chains with free or mercury-bound sulfhydryls) was 75 ± 5 gauss. On comparing the spectra of the second group of hemoproteins (75 gauss line-width) it appears that the high spin contribution of γ+PMB, Hb+A and Hb+Ft are nearly equal; that of β+PMB and γ+SH is about 60% and that of β+SH is only 27% of the high-spin contribution of Hb+A or Hb+Ft for equal heme concentration. It is inferred that the isolated γ+SH and β+SH chains are partly low-spin, the low-spin form being of a type non-observable under the experimental conditions (9.5 GHz and 77 °K).This view gains support from the visible absorption spectra of the ferri chains. At 0 °C, unlike that of Hb+A, Hb+Ft and α+SH, the spectrum of γ+SH was partly low-spin and that of β+SH much more so. The decreased high-spin content of these species seen from electron spin resonance at 77 °K may be explained as due to the effect of low temperature on a thermal equilibrium mixture of high and low-spin types.The results are discussed in the context of chain interactions in the hemoglobins.