Granulocyte‐mediated reactions such as opsonization, chemotaxis, and release of granulocyte myeloperoxidase and lactoferrin were studied in properdin‐deficient and normal human serum incubated with serogroup A and W‐135 meningococci. There were no differences between the sera when serogroup A meningococci were studied. Opsonic and chemotactic activity were impaired against serogroup W‐135 meningococci in properdin‐deficient serum. Restitution with properdin restored both activities. We found similar release of myeloperoxidase and lactoferrin from granulocytes challenged with serogroup A or W‐135 meningococci in either sera. These findings are in accordance with the clinical observations of meningococcal infections caused by serogroup W‐135 in properdin‐deficient patients as well as the absence of infections caused by serogroup A meningococci.