The nerve growth factor protein was purified over 100-fold from adult mouse salivary glands. The first step was a gel filtration on Sephadex G-100 at pH 7.5 of the aqueous gland extract. After gel filtration, most of the NGF activity was eluted in the 80,000-90,000-molecular-weight region (G-100 pool). The G-100 pool was dialyzed at pH 5.0 and fractionated by CM52 cellulose chromatography at pH 5.0. Recovery from CM52 cellulose columns was quantitative for protein and ranged 80-100 per cent for the nerve growth factor activity; the latter was almost completely carried by a protein which did not show any heterogeneity when examined by several analytical tests. The purified nerve growth factor showed an S(20,w) = 2.43, a D(20,w) = 7.30 and a 30,000 molecular weight. The over-all recovery was about 45 per cent.