Antibodies to cytomegalovirus (CMV) were determined in Thai blood donors using the complement fixation (CF) test and enzyme-linked immunosorbent assay (ELISA). A total of 203 voluntary blood donors, 181 males and 22 females, who came to the Blood Bank at Siriraj Hospital during February 1985, were investigated. Their ages ranged from 17 to 53 years (mean 24.3 +/- 6.9). Seventy-three out of 156 (46.8%) and 171 out of 203 (84.2%) sera were positive for CMV antibodies as detected by the CF test and ELISA respectively. The result of ELISA showed that 95.5 per cent of the female blood donors and 82.9 per cent of the males possessed CMV antibodies. No difference in the geometric mean titres of either sex was noted. The findings indicated that ELISA was more sensitive than the CF test for detecting CMV antibodies. The high percentage of CMV-seropositive blood donors indicates that CMV infection is common in this country. Therefore, it might be necessary to test blood donors for CMV antibodies when they are giving blood for use by certain patients, especially immunocompromised ones; the same observation applies with regard to organ donors before transplantation is carried out.