The hemoglobin found in the nucleated erythrocytes of the arcid blood clam Noetia ponderosa is heterogeneous and consists of two electrophoretic components, Hb-Major and Hb-Minor, present in about 80% and 20% proportions, respectively. Both components are hemoglobin dimers over a wide concentration range based on light-scattering measurements. No higher aggregation states are observed. The oxygen binding by Hb-Major and Hb-Minor is characterized by p50 values of 16.8 and 8.7 mm of Hg and Hill coefficients of 1.4 and 1.2, respectively, at pH 7.0 and 25 degrees C. Neither component exhibits an alkaline Bohr effect. An unusual nonlinear Hill plot is observed for Hb-Major. Hb-Major is composed of two different polypeptide chains and thus is a heterodimer based on sodium dodecyl sulfate/urea-polyacrylamide gel electrophoresis and reverse phase high performance liquid chromatography. By the same methods, Hb-Minor is a homodimer and may share a common chain with Hb-Major. Amino acid compositions of the two hemoglobins indicate 2 histidines/polypeptide chain which are presumably involved in the coordination of the heme iron. Visible absorption spectra indicate the heme environment is normal in the oxy state but perhaps more constrained in the deoxy state. Oxygen binding as a function of temperature and concentration and binding by the intact erythrocytes indicates the absence of intracellular regulators of oxygen binding.