In a study of a Caucasian population in Western Australia the prevalence of antibodies to Epstein-Barr virus (EBV) was 41% in the 9- to 10-year age group, 80% in the 16 to 19-year age group and 92% in young adults. The age-specific annual seroconversion rates indicated two peaks of primary EBV infection in the population studied - one under 5 years of age and the other at adolescence. The geometric mean titre rose with age, from 23 at 5-6 years to 53 at 36-40 years. It was shown that in 73 families studied there was evidence of probable spread of EBV infection among siblings, particularly between those of the same sex. Serological study of patients with infectious mononucleosis indicated that 100% of those examined had antibody to EBV and the geometric mean titre was elevated to 210. Rising titres and seroconversion was demonstrated in these patients together with successful establishment of EBV-carrying cell lines from the peripheral blood in two-thirds of the cases.