We have analyzed the involvement of IL 1 in a cellular interaction during which murine B cells serve as antigen-presenting cells for a T cell clone (D10.G4.1). This T cell clone was chosen for study because it is highly dependent on exogenous IL 1 for concanavalin A (or anti-receptor antibody)-induced proliferation. We observed that B cells presented either self I-Ak plus nominal antigen or allogeneic I-Ab, and consequently induced proliferation of this T cell clone without any requirement for exogenous IL 1. After an overnight co-culture with allogenic B cells, we detected an IL 1-like activity in lysates of the cell mixture, but were unable to detect a similar activity in the culture medium. This IL 1-like activity was induced in a MHC-restricted manner. We determined that this intracellular IL 1-like activity was derived from the D10.G4.1-responding T cells, rather than from the antigen-presenting B cells. Moreover, we were unable to detect an IL 1-like activity in murine B cells after a variety of stimuli including LPS, anti-Ig, activated T cell supernatants, or combinations of these stimuli. Hybridization of cytoplasmic RNA with an oligonucleotide probe for human IL 1-alpha confirmed the T cell source of the biological activity. The IL 1-like factor derived from the cell culture mixture lysate showed two broad peaks of activity after gel filtration, one in the 10,000 to 20,000 dalton range and the second in the 35,000 to 45,000 dalton range. This first description of IL 1 activity produced by a T cell may introduce a new element to the early events leading to T cell activation and proliferation.