This chapter describes the purification of two forms of protein phosphatase- 1 (PP-I 1 and PP-l G ), and three forms of protein phosphatase-2A (PP-2A 0 , PP-2A 1 , and PP-2A 2 ) from skeletal muscle. Procedures for isolating the free catalytic subunits (termed PP-l c and PP-2A c ) are documented and the structures of these enzymes are summarized in the chapter. The free catalytic subunits are isolated by a modification of the procedure of Lee and co-workers. Skeletal muscle extracts prepared from 3000 g of muscle are adjusted to pH 7.2 with 10 M ammonium hydroxide, and 350 g of solid ammonium sulfate are added to each liter of solution to bring the degree of saturation to 55%. After standing for 30 min the suspension is centrifuged for 40 min at 4200 g and the supernatant discarded. The PP-2A c from poly(L-lysine)-Sepharose is purified in an identical manner to PP-I c .