X-linked immune deficient (XID) mice are susceptible to infection with Streptococcus pneumoniae because they fail to mount an immune response to the immunodominant phosphocholine (PC) epitope on the bacterial cell wall. It is difficult to induce PC-specific antibodies in XID mice because PC-specific B cells expressing the T15-, M167- and M603 idiotype (Id), which provide protection against S. pneumoniae, are deleted in these mice via an antigen-specific, receptor-mediated process. In addition, the standard PC hapten, p-diazophenylphosphocholine (DPPC), induces high affinity phenylphosphocholine (PPC)-specific antibodies in XID mice, which are not protective against S. pneumoniae. We have used a novel PC hapten, p-nitrophenyl-6-(O-phosphocholine)hydroxyhexanoate (EPC), to induce PC-specific antibodies in XID mice. The immune response to EPC-keyhole limpet hemacyanin (KLH) is dominated by IgG1, VH1+, T15-Id-, PC-inhibitable antibodies. A small IgM anti-PC response having a consistent T15-Id+ component is also induced in XID mice, whereas normal mice produce a large IgM response dominated by T15-Id+ antibodies. The immune response to EPC-KLH remains predominantly PC-inhibitable even after multiple immunizations, while the response to DPPC-KLH becomes dominated by PPC-specific antibodies. C.CBA/N mice immunized twice with EPC-KLH are protected against 10(4) S. pneumoniae while as few as 10 bacteria are 100% lethal for the unimmunized controls. The ability of EPC-protein to induce a long-lived, PC-specific response should make this hapten a potential TD vaccine candidate for S. pneumoniae.