A complex containing the minor coat protein or adsorptionprotein (A protein) of bacteriophage fl has been solubilized from the fl virion, using the detergent deoxycholate. This complex was resolved from the fl DNA and from the fl major coat protein, or B protein, by gel filtration in the presence of deoxycholate. The A protein complex migrated as a single band on sodium dodecyl sulfate-urea-polyacrylamide gels corresponding to a molecular weight of 60 000. Analysis of the amino acid composition and amino terminal residues of this preparation indicates that the preparation contains a 20% contamination of additional protein species. Antibody against purified fd A protein is cross-reactive with deoxycholate-purified fl A protein and with fl phage. Electron microscopic observation of negatively stained complexes of fl phage with this anti-fd A protein antibody and ferritin conjugated goat anti-rabbit IgG antibody revealed phages with ferritin particles at their termini or complexes of two or more phages joined together at one end by ferritin, indicating that the complex of A protein molecules is located at one end of the filamentous fl virion.