The x-linked CBA/N defect results in an altered idiotype expression among the anti-phosphocholine (PC) antibodies produced after antigenic challenge with the thymic dependent antigen PC-KLH but does not preclude the response to this hapten as previously suggested. The majority of immune-defective F1 male mice can be divided into 2 groups based on their T15 idiotype profile. Group 1 mice fail to produce anti-PC antibodies bearing the T15 idiotype in either a primary or secondary response, whereas group 2 mice produce low levels of T15 idiotype; however, this idiotype often appears only after secondary immunization. These responses are distinct from the anti-PC response of normal F1 females, which is predominantly of the T15 idiotype. In addition to the altered idiotype expression, F1 male mice exhibited a greatly reduced primary anti-PC response compared to normal mice, and secondary responses were approximately one-third that of normal mice. The delayed expression of anti-PC antibodies in immune defective mice appears to be due to their inability to produce IgM anti-PC antibodies in either a primary or secondary response to PC-KLH.