Bacteriophage T4 carrying an amber mutation in gene 22 plus an amber mutation in gene 21 form aberrant, tubular structures termed rough polyheads, instead of complete phage when they infect Escherichia coli B. These rough polyheads consist almost entirely of the major capsid protein in its uncleaved form (gp23). When rough polyheads are treated under mild conditions with any of the five proteases, trypsin, chymotrypsin, thermolysin, pronase, or the protease from Staphylococcus aureus V8, the gp23 is rapidly hydrolyzed at a limited number of peptide bonds. In contrast, cleaved capsid protein (gp23) in mature phage capsids is completely resistant to proteolysis under the same conditions. A major project in this laboratory requires determining the primary structure of gp23, a large protein (Mr = 58,000) quite rich in those amino acids at which cleavages are achieved by conventional means. Recovery of peptides from the complex mixtures resulting from such cleavages proved to be extremely difficult. The limited proteolysis of gp23 in rough polyheads had yielded a set of large, easily purified fragments which are greatly simplifying the task of determining the primary structure of this protein.