Agrobacterium tumefaciens is a Gram-negative rod able to induce crown-gall-like tumors in the potato discs. The cytotoxicity of extracts from Tabebuia and Xetospongia spp. was evaluated using this model. Two out of the six extracts tested (dichloromethane- and partial methanol-soluble extracts of Xetospongias spp.) showed the highest biological activity. Keywords: Tabebuia spp., Xetospongias spp., Agrobacterium tumefaciens, Ti plasmidum, Solanum tuberosum and, cytotoxicity