A case of anaphylaxis during autologous bone marrow infusion is reported. The patient was demonstrated to be skin test positive to fetal calf serum used in the cryopreservation of his bone marrow cells. The patient's serum was demonstrated to contain IgE antibody directed against bovine serum albumin. A second aliquot of the patient's bone marrow preparation was depleted of contaminating bovine proteins, and the patient successfully received a transplant and was engrafted. Clinicians need to be aware that the increasing use of biologic response modifiers, both as whole cells and effector molecules manipulated or produced in vitro, may lead to cryptic reactions to xenogeneic proteins.