Analytical observations have been made with the air ultracentrifuge on concentrated staphylococcus bacteriophage solutions and on these solutions inactivated by alkali, chymo-trypsin, and heat.

All active solutions contain a homogeneous heavy component that sediments with a constant of s20° = ca. 650 x 10–13 cm. sec.–1 dynes–1, has an apparent density of ca. 1.20, and a molecular weight probably not less than 200 millions. There is also present some very light ultraviolet-absorbing material which is not a carrier of bacteriophage activity. The amount of the heavy component is not strictly proportional to the bacteriophage activity so that if the activity resides in it, as appears to be the case, inactivation may occur without measurable change in molecular size and shape.

When the bacteriophage solutions are inactivated by chymo-trypsin, the heavy component is not disrupted but the sedimenting boundaries have always been fairly diffuse. As the activity gradually disappears from alkaline solutions, the heavy component is replaced by unsedimentable material. When a solution is inactivated by heating, a dilute gel is produced which sediments with an exceptionally sharp boundary in a relatively intense centrifugal field,

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