A novel form of phosphocholine (PC), p-nitrophenyl-6-(O-phosphocholine)hydroxyhexanoate (EPC) coupled to keyhole limpet hemocyanin (KLH) has been compared with unencapsulated, avirulent Streptococcus pneumoniae (R36a) and with the traditional thymus-dependent form of PC, diazophenylphosphocholine (DPPC)-conjugated KLH for its vaccine potential against virulent S. pneumoniae. Immunization with any of these three PC-containing Ags protects normal mice against a lethal challenge with 10(4) S. pneumoniae, whereas only EPC-KLH provides total protection to Xid mice. DPPC-KLH and unencapsulated S. pneumoniae confer less than 40% protection in Xid mice. Passive transfer of a PC-specific hybridoma Ab made from EPC-KLH-immunized Xid mice also provided protection against lethal challenge with S. pneumoniae. Protective anti-PC Ab were capable of binding to the surface of virulent bacteria, whereas anti-PC Ab incapable of binding to the bacterial surface failed to protect. Furthermore, serum Ab from EPC-KLH immunized and protected mice bound to S. pneumoniae, whereas secondary Abs from DPPC-KLH- or R36a-immunized mice failed to bind to the bacteria. EPC-KLH is potentially a vaccine candidate for pneumococcal prophylaxis in settings of immune compromise.