During attempts to generate Sendai virus-specific T-cell hybridomas, a number of autoreactive T-cell hybridomas were produced. These hybrids secrete IL-2 in response to class II-positive syngeneic cells in the absence of added Sendai virus. Class II molecules on the stimulator cells are involved since monoclonal anti-class II antibodies block the reaction, and mapping studies using recombinant mouse strains show the response is class II restricted. Hybridomas adapted to grow in serum-free medium do not respond to syngeneic cells in the absence of serum; however, addition of fetal calf serum (FCS) restores the response. The component of FCS responsible for the stimulation of the hybridomas has been partially purified and characterized, and its mode of action investigated. These findings suggest that the putative 'autoreactive' T-cell hybridomas reported by others may be specific for a component of the culture medium rather than being truly self-reactive.