Cpl-1, a pneumococcal phage lytic enzyme, was tested in rats with experimental endocarditis due to Streptococcus pneamoniae WB4. High-dose regimen Cpl-1 eliminated pneumococci from blood within 30 min and decreased bacterial titers in vegetations (> 4 log(10) CFU/g) within 2 h. Rapid bacterial lysis induced by Cpl-1 treatment increased cytokine secretion noticeably.