A glycanase activity, catalyzing the depolymerization of host capsular polysaccharide, is associated with Escherichia coli capsule bacteriophage no. 29, a small virus with an isometric head, carrying a base plate with a set of spikes. The bacteriophage particles were disrupted by mild acid treatment (5 to 8 min at pH 3.5 and 37 C), and the enzymatically active fragments were isolated and subjected to sodium dodecyl sulfate-gel electrophoresis as well as to electron microscopy. Of the at least nine different polypeptide chains found in the complete virion, three (of 57,000 plus or minus 3,000, 29,500 plus or minus 2,000 and 13,500 plus or minus 1,000 daltons) were detected in detached base plates. They had the appearance of six-pointed stars of about 14 nm in outer diameter, with a central hole or prop, carrying six (or, possibly, a multiple thereof) spikes. Two sizes of polypeptide chains (57,000 and 29,500) were found in pure spikes, cylindrical particles of about 14.5 to 15 nm in length and 5 nm in diameter, and one (57,000) in -- still capsule depolymerizing -- spike subunits of roughly 5 nm in diameter. Phage 29 spike preparations, homogeneous in analytical ultracentrifugation and immunoelectrophoresis, were found to have a molecular weight of 245,000, as determined from the sedimentation equilibrium, and to contain equimolar amounts of the two polypeptides, probably three copies of each per organelle. The amino acid analysis of the isolated spikes revealed that aspartic acid, alanine, serine, and glycine are their dominant constituents; no amino sugars or other carbohydrates were detected in the preparations.