Specific proliferative T-cell responses were induced in the lymph node cells (LNC) of mice immunised with a sucrose density gradient purified preparation of respiratory syncytial (RS) virus or an immunoaffinity purified preparation of the F glycoprotein. Inhibition studies and flow cytometric analysis showed that the responding cell population were CD4+ T cells. The cytokines produced by virus-specific and F-specific cells were assessed using the CTLL cell line. Peak quantities of cytokine were consistently detected in the supernatants of stimulated cultures 24 h prior to maximum proliferation. The proportion of IL-2 released was determined by blocking IL-2 activity with an anti-IL-2 monoclonal antibody. In cultures of RS virus primed LNC challenged with whole virus there was a switch of cytokine production from 70% IL-2 at day 3 to 80% IL-4 by 6 days of culture. In contrast, LNC cultures from mice immunised with F protein secreted 75-100% IL-2 throughout the culture period. These data suggest that after 6 days of challenge with viral antigen, the RS virus-primed LNC response consists of T helper cells which are predominantly of the Th2 subset, secreting IL-4, whilst F protein-primed LNC secrete large quantities of IL-2 and can therefore be classified as predominantly of the Th1 subset.