The full genome sequence of bacteriophage SPN1S, which infects Salmonella, contains genes that encode homologues of holin, endolysin and Rz/Rz1-like accessory proteins, which are 4 phage lysis proteins. The ability of these proteins to lyse Escherichia coli cells when overexpressed was evaluated. In contrast to other endolysins, the expression of endolysin and Rz/Rz1-like proteins was sufficient to cause lysis. The endolysin was tagged with oligohistidine at the N-terminus and purified by affinity chromatography. The endolysin has a lysozyme-like superfamily domain, and its activity was much stronger than that of lysozyme from chicken egg white. We used the chelating agent, ethylenediaminetetraacetic acid (EDTA), to increase outer membrane permeability, and it greatly enhanced the lytic activity of SPN1S endolysin. The antimicrobial activity of endolysin was stable over broad pH and temperature ranges and was active from pH 7.0 to 10.5 and from 25 °C to 45 °C. The SPN1S endolysin could kill most of the tested Gram-negative strains, but the Gram-positive strains were resistant. SPN1S endolysin, like lysozyme, cleaves the glycosidic bond of peptidoglycan. These results suggested that SPN1S endolysin has potential as a therapeutic agent against Gram-negative bacteria.