Antibody titres against pneumococcal capsular and cell wall antigens and the immune response to polyvalent pneumococcal vaccine were measured in 21 splenectomized patients and 12 healthy controls. Most individuals possessed anti-pneumococcal capsular polysaccharide antibodies of IgG, IgA and IgM classes. The anti-capsular IgG was predominantly of the IgG1 and IgG2 subclasses; only occasional individuals had any detectable titre in IgG3 or IgG4 subclass. Most individuals responded to immunization with Pneumovax. There was no clear difference between groups of control and splenectomized subjects, although three of the splenectomized patients had undetectable pre-immunization anti-capsular titres in one or more subclass which failed to rise following immunization. All subjects tested had anti-phosphocholine antibodies in IgG, IgA and IgM classes with the exception of a single splenectomized patient who lacked detectable anti-phosphocholine IgM. Pre-immunization titres where similar in healthy controls and splenectomized patients. There was no demonstrable rise in anti-phosphocholine titre following immunization with Pneumovax.