A mutant at the carboxyl end of the terminal protein, p3, of phage π29 DNA has been constructed by inserting an oligonucleo-tide containing the stop translation codon TGA in the three possible reading frames, immediately downstream of a π29 DNA fragment coding for all but the last five amino acids of protein p3. The activity in the formation of the p3-dAMP initiation complex in vitro of this mutant as well as another one previously isolated, also mutated at the carboxyl end, have been tested. The results obtained suggest that an intact carboxyl end in the π29 terminal protein is essential for its normal primer function in DNA replication.