Luis Astudillo, Cristina Theoduloz and Guillermo Schmeda-Hirschmann. Laboratorio de Química de Productos Naturales, Instituto de Química de Recursos Naturales, Universidad de Talca, Casilla 747, Talca, Chile.Abstract The biological activity of extracts from the aerial parts of five Argentinian Prosopis species and the exudate of P. flexuosa were assessed for DNA binding, β-glucosidase inhibition and free radical scavenging effect using the DPPH decoloration assay. DNA binding effect was found mainly in the basic fraction. The alkaloids tryptamine as well as piperidine and phenethylamine derivatives were isolated from the basic extracts. At 0.50 mg/ml, DNA binding activities ranged from 28% for tryptamine to 0–27% for the phenethylamine and 47–54% for the piperidine derivatives. Tryptamine and 2-β-methyl-3-β-hydroxy-6-β-piperidinedodecanol showed a moderate inhibition (27–32%) of the enzyme β-glucosidase at 100 μg/ml. The exudate of P. flexuosa displayed a strong free radical scavenger effect in the DPPH decoloration assay. The main active constituent was identified as catechin.