tubes and centrifuged in an International Model PR-2 refrigerated centrifuge at 1300 g (2450 rpm) X 15 minutes. The initial supernatant (S,.) was set aside on ice. The initial precipitate (P,) was re-suspended in half the original homogenate volume of 0.32 M sucrose with seven up and down strokes, and centrifuged in the PR-2 centrifuge at 1300 g (2450 rpm) X 15 minutes. The resulting precipitate (P,W,) was saved for scintillation counting and protein determination. The supernatant from this step was combined with the supernatant from step one and centrifuged in two 12 ml graduated conical plastic test tubes in the International Model B-20 refrigerated centrifuge at 17,000 g (14,500 rpm) X 10 minutes. The supernatant (S.) from this step was saved for scintillation counting and protein determination. The precipitate from this step (Ps) was re-suspended in 0.32 M sucrose by moving the pestle manually. Now called Psr, this was layered over a discontinuous 0.32 M, 0.8 M and 1.2 M sucrose gradient in 12 ml nitrocellulose test tubes, and centrifuged in the SW 41 Rotor of the Beckman-Spinco model L-2 refrigerated ultracentrifuge at 97,500 g avo (28,000 rpm) X 60 minutes. The resulting gradient fractions were drawn off, using a suction apparatus, and saved for isotope and protein determinations. The radioactive counting procedure was as follows: 0.7 ml of the sample wa.s drawn into a new 1 ml plastic serological pipette, 0.2 ml were drained into a 13 X 100 mm tube and frozen for protein assay. The remaining 0.5 ml was washed into a counting vial with a minipet containing 2.5 ml H20. Then, 13 ml of the-I-T21 Fluor (toluene ;-PPO-:-POPOP-+-Triton X-lOO) was added to each vial so that a total vial volume could be made up to 16 ml. The tubes were tightly capped, thoroughly shaken, numbered and placed in a Nuclear Chicago Mark 1 Liquid Scintillation Counting System.