Expiants of five plant species (Allium cepa, Antirrhinum majus, Brassica campestris. Glycine max, and Nicotiana tabacum) were co-cultivated with three Agrobacterium tumefaciens strains under different conditions to assess the effects of acetosyringone and medium pH on strain virulence. Tumours were incited on all dicotyledonous species by strains N2/73 and A281. The presence of acetosyringone during co-cultivation generally enhanced the virulence of these strains, most markedly N2/73 on A. majus and G. max, and A281 on G. max. Strain Ach5 was virulent only on N. tabacum in the absence of acetosyringone, which, when present, extended the host range to include A. majus. There was evidence to suggest that acetosyringone may suppress virulence in some strain/plant species interactions. Virulence was affected in some cases by medium pH, but there was no general effect across plant species.