Aqueous extracts of Uncaria tomentosa (Willdenow ex Roemer and Schultes) DC. (Rubiaceae) ('Uña de gato'), were analyzed for the presence of toxic compounds in Chinese hamster ovary cells (CHO) and bacterial cells (Photobacterium phosphoreum). Toxicity was evaluated by four systems: Neutral red assay (NR), total protein content (KB), tetrazolium assay (MTT) and Microtox test. The extracts of U. tomentosa did not show toxicity in vitro at the concentrations tested. Testing in vitro could be a valuable tool for evaluating toxicity of medicinal plants.