Lumbricus terrestris HbO2 and HbCO dissociated below pH 5.0; a time-dependent alteration to the met form occurred at pH less than 5 and pH less than 4.5, respectively. The extent of dissociation was unaffected by alkaline earth cations but was decreased by an increase in ionic strength. HbO2 and HbCO exposed to pH 4.0-4.8 were centrifuged to obtain the undissociated pellet (P1) and dissociated supernatant (S1) fractions. S1 was reassociated at pH 7.0 by dialysis against various buffers and then centrifuged to obtain the reassociated pellet (P2) and unreassociated supernatant (S2) fractions. Reassociation was possible only if S1 was dialyzed against water prior to return to neutral pH; otherwise precipitation occurred starting at about pH 5.3. The extent of reassociation varied from about 40 to 80%, was usually higher for HbCO than HbO2, and was unaffected by an increase in ionic strength or by Ca(II). Gel filtration of P2 on Sephacryl S-300 at neutral pH gave one peak IaR, eluting at a slightly greater volume than the native Hb; S1 and S2 gave in addition, three peaks, Ib (200 kDa), II (65 kDa), and III (18 kDa). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that P2 was slightly deficient in subunit M relative to the Hb, that Ib was deficient in subunits D1 and D2 and that II and III consisted of subunits D1 + D2 + T and subunit M, respectively. Scanning transmission electron microscopy of P2 showed that it was smaller than the native hemoglobin: 25 nm in diameter and 16 nm in height, instead of 30 X 20 nm. Comparison of the results of the dissociations of Lumbricus Hb at alkaline pH (Kapp, O. H., Polidori, G., Mainwaring, M., Crewe, A. V., Vinogradov, S. N. (1984) J. Biol. Chem. 259, 628-639) with those obtained in this study suggested that the Hb quaternary structure was not multimeric and that an alternative model had to be considered. In the proposed model it is assumed that subunits D1 and D2 form a scaffolding or "bracelet," decorated with 12 complexes of M and T subunits.