Cell-mediated immunity to cytomegalovirus (CMV) was studied in a bone marrow transplant patient with evidence of active CMV infection. The lymphocytes from this patient were found to specifically recognize and respond in vitro by transformation to CMV-infected Wistar-38 fibroblasts and by production of macrophage migration inhibition factor to CMV antigen. In addition, plasma and spinal fluid from the patient were found to contain blocking factor that specifically inhibited the lymphocyte response in the above assays. Biochemical, biophysical, and immunological studies indicate that the blocking factor may be an antigen-antibody complex. The immunosuppression required for bone marrow transplantation results in an increased number of severe and often life-threatening viral infections (4, 11). Immunosuppressive regimens utilized in patients undergoing renal transplantation also result in a high incidence of viral infections, particularly infection with cytomegalovirus (CMV) (3, 8). Increasing complement-fixing antibody titers to CMV have been reported in both renal and bone marrow transplant recipients with clinical evidence of CMV infection (3, 4, 8, 9, 11). Thus, despite immunosuppression, a specific antibody response is seen and a vigorous serological response may indicate a more favorable prognosis (11). Depression of cellular immunity, as measured by blasto-genic response to phytohemagglutinin, has been described in renal transplant patients infected with CMV (9). However, the presence of specific cellular immune response to CMV in infected patients is poorly documented. This report characterizes the cell-mediated immune response to CMV in a patient who developed evidence of acute CMV infection following bone marrow transplantation. Lymphoid cells from this patient were found to specifically recognize and respond in vitro by transformation to a CMV-infected cell line and by production of macrophage migration inhibition factor (MIF) to CMV antigen. Furthermore, plasma and spinal fluid from this patient were found to contain blocking factor that specifically inhibited lymphocyte transformation and MIF production in response to CMV antigen. Partial characterization of the blocking factor indicates that it may be an antigen-antibody complex.