In the present study we have investigated the use of recombinant ovine IL-1β and TNF-α both alone and in combination, as natural adjuvants in vaccination trials in sheep. Initial experiments were conducted to investigate the physiological effects of the cytokines in vivo and determine what dose could be administered without adverse pyrogenic effects. Even at the maximum dose tested (100 μg) the only significant physiological effect was a transient increase in body temperature of approximately 2°C in sheep injected with TNF-α. Administration of either cytokine had profound effects on the levels of circulating leucocytes for up to 5 days postinjection. The incorporation of either IL-1β or TNF-α in aqueous or Al(OH)3 vaccine formulations enhanced antibody responses to a recombinant antigen from the cestode parasite Taenia ovis. The addition of IL-1β to aqueous vaccine formulations increased antibody responses 15-20-fold and in Al(OH)3 formulations by three to six fold. TNF-α stimulated 1.5 to six-fold and 2.5 to seven-fold increases in antibody levels in aqueous and Al(OH)3-based formulations, respectively, in a dose-dependent manner. The addition of either cytokine to Quil A or IFA vaccines did not enhance the antibody levels elicited. When 10 μg of both IL-1β and TNF-α were incorporated in the aqueous or Al(OH)3 vaccine formulations, increases of 21-fold and 25-fold, respectively, were observed in antibody levels. The adjuvant activity of IL-1β and TNF-α in combination in the Al(OH)3-based vaccine resulted in antibody levels commensurate with those obtained using Quil A or IFA.