Primary and secondary immunizations with live, attenuated yellow fever virus vaccine (17D strain) were performed in order to study the course of appearance of virus-neutralizing antibodies and immunoglobulin M (IgM) and IgG antibodies directed against the virus and the interferon-dependent enzyme 2',5'-oligoadenylate synthetase (2',5'AS) activity, determined in homogenates of peripheral B and T lymphocytes. From cellular ATP, this enzyme generates 2',5'-oligoadenylates which mediate degradation of viral mRNA by stimulation of a latent RNase. By day 4 after the first immunization, the earliest and highest 2',5'AS activity was present in the T-lymphocyte fraction. By day 7, the enzyme activity was highest in the B-lymphocyte fraction. Virus-neutralizing antibodies appeared on day 7, and IgM antibodies were present on day 12. After the second immunization, performed 2 years +/- 2 months later, the only significant increase in 2',5'AS activity was observed in the T-lymphocyte fraction. Virus-neutralizing antibodies were present from day 1, whereas no IgM antibodies were detected. By day 12, 80% of the vaccines were IgG positive. In the primary and secondary (memory) immune responses, 2',5'AS activity is expressed in the T-lymphocyte fraction prior to the appearance of antibodies directed against the virus and may serve as an early and sensitive marker of an ongoing virus infection which is otherwise difficult to detect. No change in conventional laboratory analysis parameters, such as in differential blood cell counts or total IgA, IgG, and IgM, disclosed the immune activity in either the primary or the secondary immunization.