• A 4-year-old girl with recurrent, severe bacterial infections and absence of both the second component of complement and galactokinase was investigated for immunodeficiency. The C2 deficiency (C2D) was diagnosed after four major pyogenic infections. Results of studies of cellular and humoral immunity were normal, as were polymorphonuclear leukocyte chemotaxis and bactericidal activities and alternative-pathway hemolytic activity. Serum chemotactic and opsonic activities were deficient in this patient and in an older, asymptomatic sibling with C2D. Fresh-frozen plasma, administered during an episode of Streptococcus pneumoniae meningitis, enhanced serum opsonic activity at 12 hours after infusion. To our knowledge, this is the first description of C2D in a patient with a documented second, unusual genetic defect.