In vitro proliferating shoots of a clone of the English elm, Ulmus pro
cera SR4, were infected with the wild-type Agrobacterium tumefaciens s
train 82.139 under a variety of conditions. A co-cultivation medium of
pH 5.2 was found to be optimal for infection by uninduced A. tumefaci
ens 82.139, with 70-75% of inoculated shoots forming tumours. In the p
resence of 0.1 mM acetosyringone (a virulence inducing agent), the hig
hest percentage tumour formation was obtained following co-cultivation
at pH 5.8. The tumours were brown to white in colour, and were able t
o regenerate shoots capable of independent growth. Some of these shoot
s proved to be transformed with the T-DNA of A. tumefaciens 82.139 on
the basis of nopaline production and Southern hybridisation. These res
ults are discussed in relation to the development of a transformation
protocol using disarmed agrobacteria harbouring manipulated Ti plasmid
s.