AbstractMeasurements of electric birefringence, sedimentation velocity, and biological adsorption rate are used to study the properties of bacteriophage T4B in the presence of excess tryptophan. The adsorption rate determined in borate buffer pH 9 (at 25°C) increases from 0.003 × 10−8ml min−1(0.025M) to 0.130 × 10−8ml min−1(0.150M). The Kerr coefficient, rotational diffusion coefficient, and the sedimentation coefficient of the phage are also dependent on buffer concentration and reach plateau values above 0.12Mgiven byKsp= −(275 ± 18) × 10−9OD−1cm2statvolt−2,D25,w= 133 ± 4 sec−1, ands20,w= 818 ± 11S. From a comparison of electric birefringence measurements of T4B and T4D it is concluded that T4D and T4B (in the presence of excess tryptophan) exhibit a similar hydrodynamic behavior. The change in physical parameters is solely due to a shift in fiber configuration. At high buffer concentrations the fibers make an angle of approximately 3π/4 with the sheath and the permanent dipole moment is about 200,000 D. This dipole moment is roughly ten times as large as that of a phage particle with nonextended fibers. This difference may be due to a change in hydrodynamic center upon fiber extension or to the presence of positive charges on the fiber tips, or both. At intermediate buffer concentrations the phage population behaves as if it were monodisperse. Probably not all six fibers are exten