The involvement of red blood cell spectrin in the ubiquitination process was studied. Spectrin was found to form two ubiquitin-associated derivatives, a DTT-sensitive ubiquitin adduct and a DTT-insensitive conjugate, characteristic intermediate and final products of the ubiquitination reaction cascade. In addition to spectrin and ubiquitin, ubiquitin-activating enzyme (E1) and ATP were necessary and sufficient to form both the spectrin-ubiquitin adduct and conjugate. No exogenous ubiquitin-conjugating (E2) or ligase (E3) activities were required, suggesting that erythrocyte spectrin is an E2 ubiquitin-conjugating enzyme able to target itself. Both ubiquitin adduct and conjugate were linked to the alpha subunit of spectrin, suggesting that the ubiquitin-conjugating (UBC) domain and its target regions reside on the same subunit.