An acidic protein with phospholipase A2 activity was purified to homogeneity from the venom of the Northeast Argentinian viperid Bothrops alternatus by two chromatographic steps: a conventional gel filtration on Sephadex G-75 and reversed phase on C18 HPLC column.A molecular mass of 14185.48Da was determined by mass spectrometry, displaying a homodimer conformation. The kinetic assay demonstrated a catalytically active phospholipase A2 in correspondence with Asp49 PLA2 group. The enzyme designated Ba SpII RP4 contains an amino acid composition of 121 residues and a calculated theoretical pI value of 4.88. Amino acid sequence alignments with other Bothrops PLA2 revealed a high degree of homology sequence (90-56%). Ba SpII RP4 did not show myotoxic activity upon muscular fibers at doses up to 100μg i.m. route injection or lethal response when it was i.p. injected at the hightest dose of 200μg. This toxin generates slight biological activities like paw edema inflammation and a delay in the clotting time, although Ba SpII RP4 exhibited catalytic activity. The primary amino acid sequence, determined a quadruple-time of flight (Q-TOF) hybrid mass spectrometer Q-TOF Ultima from Micromass (Manchester, UK) equipped with a nano Zspray source operating in a positive ion mode and tandem mass spectrum, an ESI/MS mass spectrum (TOF MS mode) " de novo amino acid sequencing" , also provides more database about the small group of the non-myotoxic PLA2s isolated up to the present. © 2010 Elsevier Ltd.