Anti-phosphocholine (PC) antibody mediated protection against many strains of Streptococcus pneumoniae, and hybridoma anti-PC antibodies protected mice from fatal infections with types 1 and 3 S. pneumoniae. Live types 1, 3, 5, 6A, and 19F S. pneumoniae had similar amounts of surface PC accessible to antibody. Furthermore, mice expressing the X-linked immunodeficiency (xid) of the CBA/N strain were found to be more susceptible to infection with S. pneumoniae of types 3, 6A, and 19F than were immunologically normal mice. The only exception to these results was with the type 5 strain, which was highly virulent for both xid and normal mice. In addition, we were unable to protect mice against infection with the type 5 strain by using anti-PC antibody.