Granulocytes, monocytes, and T- and B-lymphocytes were separated from 28 blood samples collected from 5 bone marrow transplant (BMT) recipients. About 40% of granulocyte, monocyte, and B-lymphocyte samples were CMV DNA-positive by polymerase chain reaction in recipients with cytomegalovirus (CMV) infection. CMV DNA was rarely detected in separated T-lymphocytes. Within each of the simultaneously separated paired samples, there were several with single positive cell subtypes. Monocytes, granulocytes, and B-lymphocytes were the single positive samples in some instances. Thus, it is important to have all of the different cell subtypes present in samples for detection of CMV DNA in peripheral blood. We also studied the appearance of CMV DNA in plasma and peripheral blood leukocytes (PBLs) from 351 blood samples collected from 30 BMT recipients during a follow-up period of at least 3 months after BMT. All cell subtypes were represented in the PBL samples. In the 13 recipients who developed symptoms possibly associated with CMV infection or CMV disease, a correlation with the detection of CMV DNA in < or = 2 x 10(5) PBLs was found. In PBLs from 11 of the 13 BMT recipients, CMV DNA was detected before the onset of symptoms. CMV DNA was not detected in < or = 2 x 10(5) PBLs from recipients without CMV infection. The virus load in PBLs decreased during ganciclovir treatment. Nine of the 13 recipients displayed PCR-positive plasma samples, and CMV DNA was detected frequently after the onset of symptoms.