Antibody titers were measured by counterimmunoelectrophoresis in eight patients with Pseudomonas aeruginosa and in eight patients with Staphylococcus aureus bone and joint infections. Titers were obtained at the beginning of therapy in all patients with pseudomonas infections and at various intervals during and after completion of therapy in seven patients. These patients were followed clinically, with 99mTc-MDP bone and 67Ga scans, and with serial erythrocyte sedimentation rates. Six of eight of these patients had detectable pseudomonas antibodies. A fall in antibody titer occurred in all six successfully treated antibody-positive patients. By contrast, none of the control patients with staphylococcal infections had antibodies to pseudomonas heptavalent antigen. The predictive value of a positive test was 100% and of a negative test, 80%. Counterimmunoelectrophoresis is a helpful adjunctive test in supporting the diagnosis and documenting a successful treatment of chronic pseudomonas bone or joint infections.