When different numbers of leaf micro-organisms were applied in droplets to non-sterile beetroot leaves inhibition of germination of Botrytis cinerea conidia was positively correlated with uptake of amino acids. Of the micro-organisms tested two isolates of Pseudomonas and an isolate of Sporobolomyces were most effective at inhibiting germination and removing amino acids from droplets at comparatively low cell densities. Leaves bearing a natural microflora needed to be wetted for 24 h before appreciable uptake of amino acids occurred and conidia of B. cinerea were inhibited from germinating. Negligible quantities of 14C-labelled amino acids or glucose, added to droplets placed on the surface of sterile beetroot leaves, were taken up by the leaf. Most of the 14C label lost from sterile 14C-labelled beetroot leaves was detected in droplets shortly after placing them on the surface of the leaves. Germination of conidia of Phoma betae and Cladosporium herbarum was greatly inhibited on beetroot leaves in the presence of a leaf surface bacterium, Pseudomonas sp. isolate 14, and there was a close relationship with uptake of amino acids by the bacterium on leaves. Percentage germination of conidia of Colletotrichum dematium f. sp. spinaciae was not inhibited by the bacterium but length of germ tubes was reduced. Presence of the bacterium stimulated development of appressoria.