SUMMARYThe uptake of manganese by barley plants grown in solution culture is stimulated in the presence of micro-organisms, irrespective of whether they originated as casual laboratory contaminants or as rhizosphere micro-organisms in the soil. The microbial effect is mediated by a watersoluble substance but neither its identity nor its mode of action has yet been established.When manganese is supplied as EDTA chelates these appear to be broken down at or near the surface of the roots and this process is accelerated in the presence of micro-organisms. Under the conditions of these experiments EDTA was absorbed by the plants much less readily than manganese.