With new techniques 19 bone marrow transplant (BMT) recipients were monitored for lymphocyte proliferation and specific IgG production in vitro by cytomegalovirus (CMV) antigen in solid phase. Twelve patients got a reactivated CMV infection as defined by virus isolation or serum IgG conversion. Lymphocyte proliferation and in vitro IgG production responses were significantly stronger in these 12 patients than in seven without ongoing CMV infection (P = .02). CMV infection was indicated by the lymphocyte responses at a mean of 45 days after BMT as against a mean of 79 days that passed before CMV growth in culture was detected (P less than .05). Lymphocyte proliferation and in vitro IgG production may thus be used as tools for diagnosis and for monitoring of CMV infections in BMT recipients.