Using an immunodiffusion assay, we tested all of the blood units donated at the Fukuoka Red Cross Blood Center from June 1991 to July 1994 for B19 antigen. Over this 3-year trial period, we detected 16 viremic cases out of approximately 560,000 blood donors. Interestingly, most of the viremic donors (15 out of 16) were detected between February 1992 and January 1993, which coincided with a local erythema infectiosum epidemic in the Fukuoka area (December 1991 to August 1992). In particular, we detected 4 cases of viremia in March 1992, which was the peak of the erythema infectiosum epidemic. The incidence of B19 viremia in this peak period was approximately 1/4,000. The viremic donors ranged in age from 17 to 45 years, and most (11/16) were between 31 and 39 years old. By ELISA, using virus particles purified from viremic donor plasma as antigen, we analyzed the prevalence of B19-specific antibody among blood donors. The antibody-positive rate was approximately 40% in donors 16-30 years old, gradually increased in middle age, and reached a peak of 92% in donors more than 61 years old.