The peptidoglycan recognition protein (PGRP) family is conserved from insects to mammals and is involved in immune regulation and bacterial clearance. They form at least three functional classes; receptors required for immune gene expression; amidases that degrade peptidoglycan and scavenge the tissues from immune-stimulating peptidoglycan; and as proteins with antibacterial activity. We here report that PGRP-SB1 is an N-acetylmuramoyl l-alanine amidase, which (in contrast to the previously described PGRP-amidases) shows antibacterial activity. PGRP-SB1 is highly active against peptidoglycans that have a diaminopimelic acid (DAP) residue in the cross-linking peptide, but lack activity to most lysine-containing peptidoglycans. The antibacterial activity is pronounced against Bacillus megaterium with an LD(50) of 1.5microg ml(-1). The bactericidal effect of PGRP-SB1 is dependent on its enzymatic activity, as the zinc co-factor is essential. The bactericidal mode of action is thus different from non-enzymatic vertebrate PGRPs that have been reported to be antibacterial.