The results from several experiments demonstrated that an effective vaccine for contagious caprine pleuropneumonia could be made with inactivated F38 mycoplasma. Evaluation of the amounts of lyophilised F38 mycoplasma plus saponin showed that the optimum formulation was 0.15 mg of mycoplasma in saponin. Saponin inactivates the mycoplasma and provides the adjuvant effect necessary to stimulate a protective immune response. The lyophilised F38 mycoplasma could be stored for 14 months at either 4 degrees C or 22 degrees C without losing its immunising potential. A single immunisation with the optimum formulation produced a protective immune response in goats that lasted for longer than one year.