The in vitro antiviral activity of antimicrobial cationic peptides: cecropin A, melittin, magainin I and II and indolicidin against the arenavirus Junin virus (JV), and herpes simplex virus type 1 (HSV-1) and 2 (HSV-2) was evaluated. Cecropin A effectively inhibited JV multiplication and failed to affect HSV replication whereas melittin impeded the multiplication of JV and HSV, but was highly toxic for the host cell. Magainins I and II exhibited inhibitory action toward HSV-1 and HSV-2 but were inactive against JV. Only indolicidin showed a direct inactivation effect on cell-free virus stocks. Besides its inhibitory effect on JV replication cecropin A also was active against the arenaviruses Tacaribe and Pichinde, mainly affecting late events of arenavirus multiplication cycle by preventing viral morphogenesis and egress from infected cells.