The inclusion of exudates from the roots of peas and oats in liquid media has been found to increase the growth of micro-organisms isolated from both control soil and the rhizosphere of three-week old pea plants. In most cases the stimulation was greater for organisms from the rhizosphere than for those from outside this zone. For some organisms, yeast extract exhibited similar growth promoting properties as did pea root exudate, but for others the exudate stimulated growth in the presence of yeast extract. The root exudate could not be completely replaced by glucose, soil extract, vitamin-free casamino acids, or a synthetic mixture of known growth factors.