Technical Abstract: The purpose of this study was to determine if larkspur toxins conjugated to protein carriers would promote active immunity in mice. Mice were injected with several larkspur toxin-protein conjugates or adjuvant alone to determine if the resulting immunologic response altered animal susceptibility to methyllycaconitine, the major toxic larkspur alkaloid. Although vaccinations appeared to provide a mild protective effect against intravenous methyllycaconitine toxicity, overlapping confidence intervals did not provide evidence of differences between the vaccinated and control groups. In the lycoctonine conjugate (LYC) vaccinated group, mouse survival was related (P = 0.001) to serum titers for methyllycaconitine doses up to 4.5 mg/kg BW. When mice with low antibody titers were removed from the vaccinated groups in which titer was related to survival, the recalculated LD50s were 20% greater than the LD50s of the control group. These results suggest that vaccination altered methyllycaconitine toxicity in mice and that vaccination may be useful in decreasing the effects of larkspur toxins in animals. Additional studies are warranted to continue development of potential larkspur vaccines for livestock.