INCLUSION CRITERIA:

All patients must have a diagnosis of X-linked hyper IgM syndrome confirmed either by molecular analysis of the CD40L gene or by flow cytometry analysis demonstrating the failure of CD40L expression on activated T cells, and/or clear X-linked inheritance (with multiple affected males) in association with defective CD40L expression.

Age greater than or equal to 4 years

Patient and or parent (for children under the age of 18) must be able to understand and sign informed consent.

Life expectancy of greater than 6 months.

Average ANC of greater than 250 cells/microL measured over 3 days during the week prior to planned administration of rhuCD40L.

EXCLUSION CRITERIA:

Serious ongoing opportunistic infection.

Use of immune-based therapies other than IVIG such as corticosteroids (doses of prednisone greater than 0.4 mg/kg/d for more than 4 weeks within the 6 months prior to enrolling in the study or any use of corticosteroids equivalent to greater than or equal to 5 mg of prednisone at the time of enrollment) or other immunomodulating drugs within 6 months prior to enrollment in the study.

Current use of other investigational drugs.

Chronic liver disease or any confounding medical illness that in the judgement of the investigators would pose added risk for study participants (e.g. cancer, severe allergies, chronic renal or pulmonary disease).

SGOT, SGPT greater than 2 times normal range; and creatinine greater than 2.0 times normal

ANC less than 250/microL; Platelets less than 50,000/microL; Hematocrit less than 25