The number of tumours developing on primary pinto bean leaves (Phaseolus vulgaris L. variety Pinto) inoculated with Agrobacterium tumefaciens (Smith and Town.) Conn, strain B6, was increased by as much as 100 per cent through the addition of several plant-growth factors and antagonists of some of these factors to the freshly inoculated leaves. Naphthylacetic acid, gibberellic acid, (2-chloroethyl) trimethylammonium chloride, and adenine gave a biphasic response with optimal promotions of tumour initiation at 10-5 to 10-4 mg/ml. Tri-iodobenzoic acid and 4-chlorophenoxyisobutyric acid were most active at 10-1 mg/ml. Mean tumour diameter showed a direct correlation with tumour number in these experiments. The results show tumour initiation to be sensitive to growth factor changes, possibly through a heightened traumatic response of the wounded leaves. The growth of the tumours was unaffected by these additions except as they altered tumour number and this secondarily affected tumour growth.