Fractionation of a lysate of bacteriophage T7-infected E. coli. Particles in a 6-liter lysate of bacteriophage T7-infected E. coli BL21 were precipitated with polyethylene glycol, clarified by low speed centrifugation and DNase-digested. (a) The particles were then layered on a cesium chloride step gradient and centrifuged . Light scattering from particles in the gradient was photographed; a bacteriophage particle band is indicated by ϕ. (b) Particles in the bracketed region of the step gradient were brought to a density of 1.367 g/ml with cesium chloride and centrifuged to equilibrium in a Beckman SW55 rotor at 42,000 rpm for 22.0 hr. at 4°C. Light scattering from the gradient was photographed. Fractions were dialyzed against 0.2 M NaCl, 0.01 M Tris-Cl, pH 7.4, 0.001 M MgCl2. The dialyzed IC fraction was the sample used for electron microscopy in Figures 3 and 4, below.