IgG class antibodies to the early antigen complex (EA; D + R components) of the Epstein-Barr virus (EBV) were found by indirect immunofluorescence in titres greater than or equal to 1:10 in 196 (49.4%) out of 397 blood donors and titres greater than or equal to 1:20 in 84 (21.2%) of these subjects. Anti-EA titres of greater than or equal to 1:2560 were detected in 6 sera, anti-EA(D) only in 17 donors (4.3%). Additional EBV-specific antibody tests were performed in sera with anti-EA-IgG titres of greater than or equal to 1:20 (n = 84), including IgM class antibodies to virus capsid antigen (anti-VCA-IgM). Specific IgM revealed active EBV infection in 12 blood donors. There was no correlation between the presence of anti-VCA-IgM and the magnitude of anti-EA-IgG titres. Therefore, it seems to be impossible to define the threshold titre for EA antibodies indicating active EBV infection. For this purpose probably a titre increase should be demonstrated in paired sera.