In an attempt to define the role of neutrophils in immunity to Naegleria fowleri in vivo, we examined the effects of treating immunized (with amoeba culture supernatant antigen) mice with the monoclonal antibody NIMP-R10, which binds to neutrophil complement receptor type 3bi (CR3) and causes selective neutrophil depletion in mice. Mice in the nonimmunized group challenged with amoebae all died by day 12, while 97% in the immunized group survived. By contrast, the immunized group treated with NIMP-R10 showed only 25% survival. The immunized group treated with "control" mouse ascites, WEM-G11, was highly resistant (90% survival). There was a significant neutrophil response in the nasal mucosa and olfactory lobes of immunized, NIMP-R10-treated mice, despite a marked degree of neutropenia similar to that seen in immunized, untreated mice. Nonimmunized mice showed virtually no neutrophil response. Despite this response in the NIMP-R10-treated mice, amoebic proliferation was not depressed, and there was no evidence of neutrophil degranulation or amoebic killing, despite the close apposition of large numbers of neutrophils to amoebae. The results indicate that neutrophils are necessary for the expression of immunity to N. fowleri.