Seven black patients with mild hereditary elliptocytosis (HE) from five unrelated families were studied. The erythrocytes of these patients exhibited an abnormal thermal sensitivity (between 45 degrees C and 47 degrees C instead of 49 degrees C). An important defect of spectrin dimer self-association was detected in two ways: (1) the proportions of spectrin dimer (SpD) extracted from membranes at 4 degrees C under low ionic strength conditions were increased between 25% and 56% (normal value 15% +/- 2%); (2) the spectrin dimer----tetramer conversion in solution were defective with an association constant value between 0.4 and 2.4 X 10(5) M-1 for a normal value of 6 +/- 0.4 X 10(5) M-1. Spectrin (Sp) from HE patients and normal volunteers (32 black and 22 white subjects) was submitted to limited tryptic digestion, followed by one- or two-dimensional separation of the peptides. Peptide patterns of crude Sp from all seven HE patients exhibited a marked and reproducible decrease in 80,000-dalton peptide (previously identified as the dimer- dimer interaction domain of the alpha-chain) and a concomitant appearance of a novel 65,000-dalton peptide. A minor fragment at 28,000 daltons was also decreased. Tryptic digestion of HE spectrin dimer and tetramer (SpT), isolated after the SpD self-association procedure in solution, revealed modifications (decrease in the 80,000-dalton peptide and presence of a 65,000-dalton peptide) predominantly in HE SpD when peptide patterns of HE SpT were quite similar to control SpT patterns. Immunoblots with anti-alpha-chain antibodies revealed that the 65,000- dalton peptide derived from the alpha-chain. Kinetic studies of Sp digestion showed that the 65,000-dalton peptide did not result from further digestion of a 74,000 intermediate and was not a precursor of 46,000- to 50,000-dalton peptides. These results show a new structural defect of Sp-alpha-chain, associated with a defective Sp dimer self- association in HE.