T he first known human kindred with hereditary deficiency of the fifth component of complement (C5) was documented in the accompanying report. This study examines several biological properties of C5-deficient (C5D) human serum, particularly sera obtained from two C5D homozygotes. The proband, who has inactive systemic lupus erythematosus is completely lacking C5, while her healthy half-sister has 1-2% of normal levels. Both sera were severely impaired in their ability to generate chemotactic activity for normal human neutrophils upon incubation with aggregated human gamma-globulin or Escherichia coli endotoxin. This function was fully restored in the sibling's serum, and substantially improved in the proband's serum, by addition of highly purified human C5 to normal serum concentrations. Sera from eight family members who were apparently heterozygous for C5 deficiency gave normal chemotactic scores. The ability of C5D serum to opsonize Saccharomyces cerevisiae (baker's yeast) or Candida albicans for ingestion by normal neutrophils was completely normal. In addition, C5D serum was capable of promoting normal phagocytosis and intracellular killing of Staphylococcus aureus. The proband's serum was incapable of mediating lysis of erythrocytes from a patient with paroxysmal nocturnal hemoglobinuria in both the sucrose hemolysia and acid hemolysis tests, and also lacked bactericidal activity against sensitized or unsensitized Salmonella typhi. The sibling's serum, containing only 1-2% of normal C5, effectively lysed S. typhi, but only at eightfold lower serum dilutions as compared to normals. These findings underscore the critical role of C5 in the generation of chemotactic activity and in cytolytic reactions, as opposed to a nonobligatory or minimal role in opsonization, at least for the organisms under study.