The pharmacokinetics of an intravenous immunoglobulin (IVIG), Gammagard (Baxter Healthcare Corp., Glendale, CA), were measured in 31 cytomegalovirus (CMV) antibody negative bone marrow transplant (BMT) patients as part of a multicenter efficacy trial of 2 weekly dose regimens. Since all patients lacked antibody to CMV and received only screened CMV negative blood products, the half‐life of the exogenous CMV antibody could be measured with an ELISA assay. The CMV antibody titer was related to the immunoglobulin concentration using a standard curve. Compared with the 22‐day half‐life in normal subjects, the half‐life in BMT patients was approximately 6 days for either the 250 mg/kg or 500 mg/kg dose regimen. The half‐life did not change over the subsequent 3 weekly doses. Peak concentrations were 3.5 ± 1.4 and 2.6 ± 0.7 mg/mL of IVIG in week 1 as well as 5.5 ± 2.6 and 3.4 ± 1.2 mg/mL in week 3 after the 250 mg/kg and 500 mg/kg, respectively. Total body clearance of IVIG was 0.61 and 0.46 mL/kg/hr for the 500 mg/kg and 250 mg/kg, respectively.