This second article in the miniseries Practical Aspects of Ambulatory Diagnosis and Management of Immunodeficiency Disorders' extends the discussion on evaluation of individuals with suspected humoral immunodeficiency by reviewing the logistics and interpretation of the patient's ability to produce antibodies to polysaccharide antigens, specifically pneumococcal surface polysaccharides. The response to these polysaccharides is important in the evaluation of patients with documented immune abnormalities and those individuals who have normal total immunoglobulin levels. Although profound immune deficiencies, such as X-linked agammaglobulinemia and severe combined immunodeficiency, are always associated with a defect in specific antibody production, some immune disorders may have variable responses, whereas others with persistent IgG or IgG subclass deficiencies may have normal or clearly abnormal antipolysaccharide antibodies. Measurement of the response to pneumococcal polysaccharides is preferred because of the availability of a pure polysaccharide vaccine for antigen challenge and standardized techniques to measure specific antibody responses.