Naturally occurring cationic antimicrobial peptides (CAPs) are an essential component of the innate immune system of multicellular organisms. At concentrations generally higher than those found in vivo, most CAPs exhibit strong antibacterial properties in vitro, but their activity may be inhibited by body fluids, a fact that could limit their future use as antimicrobial and/or immunomodulatory agents. In the present study, we evaluated the effects of human serum components on bactericidal activity of the human beta-defensin 3 (hBD-3), a CAP considered particularly promising for future therapeutic employment. Human serum diluted to 20% strongly inhibited the bactericidal activity of the peptide against both the Gram-positive species Staphylococcus aureus and the Gram-negative species Acinetobacter baumannii. Such activity was not restored in serum devoid of salts (dialyzed), pre-treated with protease inhibitors, or subjected to both of these treatments. The addition of physiological concentrations of NaCl, CaCl2, and human albumin in the bactericidal assay abolished bactericidal activity of hBD-3 against S. aureus, while it only partially inhibited the activity of the peptide against A. baumannii. Although a proteolytic activity of serum on hBD-3 was demonstrated at the protein level by Western blot, addition of physiological concentrations of trypsin to the bactericidal assay only partially affected the antibacterial properties of the peptide. Altogether, these results demonstrate a major role of mono-divalent cations and serum proteins on inhibition of hBD-3 antibacterial properties and indicate a relative lack in sensitivity of the bactericidal activity of this peptide to trypsin and trypsin-like proteases.