Some inactive precursor proteins, or zymogens, contain small, amino terminus, homopolymeric regions of Asp that neutralize the cationic charge of the active protein during synthesis. After posttranslational cleavage, the anionic propeptide fragment may exhibit antimicrobial activity. To demonstrate this, ovine trypsinogen activation peptide, and frog (Xenopus laevis) PYL activation peptide, both containing homopolymeric regions of Asp, were synthesized and tested against previously described surfactant-associated anionic peptide. Peptides inhibited the growth of both gram-negative (MIC, 0.08 to 3.00 mM) and gram-positive (MIC, 0.94 to 2.67 mM) bacteria. Small, anionic, and charge-neutralizing propeptide fragments of zymogens form a new class of host-derived antimicrobial peptides important in innate defense.