1. A medium suitable for the growth of excised pea roots has been described. This medium contains 4 per cent sucrose in addition to inorganic salts. 2. Freshly excised pea root tips 3-4 mm. long grow 60 to 95 mm. in this medium in the first week; 200 mm. or more in the first two weeks. 3. Yeast extract (0.01 per cent) must be added to the basic medium if good growth in passages after the first is to be obtained. Roots have been kept in culture with weekly transfers for four months on the basic medium with added yeast extract. 4. Crystalline vitamin B1 is capable of partially replacing yeast extract; vitamin B1 alone is not able to support the continued optimal growth of excised pea roots in later passages. 5. A mixture of pure, crystalline amino acids is capable of replacing that portion of the activity of yeast extract which is not due to vitamin B1. With the use of vitamin B1 and this mixture of amino acids a highly satisfactory, completely known medium for the cultivation of excised pea roots has been ...