The streptococcal C5a peptidase removes a six-amino-acid fragment from human C5a and thereby inactivates this chemotaxin. We used transposon and chemical mutagenesis to generate mutants of Streptococcus pyogenes that did not produce C5a peptidase. These mutants showed no alteration in expression of capsule, M protein, streptolysins O and S, or pyrogenic exotoxin C. Serial passage of a peptidase-producing strain in vivo resulted in a 100-fold increase in production of C5a peptidase. The presence of C5a peptidase delayed the accumulation of polymorphonuclear leukocytes (PMNLs) in the peritoneal cavities of mice after intraperitoneal challenge. However, there was no difference in virulence (as evaluated by LD50) between strains that produced and those that lacked C5a peptidase. Although C5a peptidase is expressed on the cell surface, antibody to this enzyme did not opsonize streptococci for phagocytosis in vitro. These studies show that C5a peptidase alters the normal host inflammatory response by delaying the accumulation of PMNLs at the foci of streptococcal infection.