The basis of the adjuvant effect of IL-1 on the clonal expansion of Ag-specific CD4 T cells was measured in vivo using the TCR transgenic T cell adoptive transfer method. Injection of Ag plus IL-1 caused naive CD4 T cells to proliferate to a greater extent than did injection of Ag alone. The T cells had to express CD28 to experience the beneficial effect of IL-1 whereas the host had to express the type I IL-1R, raising the possibility that IL-1's adjuvant properties were related to induction of CD28 ligands on APC. Surprisingly, however, expression of CD28 ligands on DC and B cells was not affected by IL-1. Therefore, the adjuvant properties of IL-1 depend on a basal level of CD28 signaling but cannot be explained by enhanced CD28 signaling due to CD28 ligand induction.