Counterimmunoelectrophoresis and gel-diffusion techniques were used to study the development of teichoic acid antibodies in eighteen patients with osteomyelitis and seven patients with septic arthritis caused by Staphylococcus aureus. Nine of eleven patients with acute osteomyelitis and three of seven with chronic osteomyelitis had a positive response to tests for teichoic acid antibody. However, only two of seven patients with septic arthritis generated a positive teichoic-acid antibody response. In two patients the test was extremely valuable in the diagnosis and management of osteomyelitis. Antibody detection appears to be a sensitive test for detecting staphylococcal osteomyelitis, especially the acute variety. It does not appear to be a reliable test for septic arthritis. It is also useful for the detection of antibody in patients who had received prior antibiotic therapy, yet have persistent foci of infection. The presence of the teichoic acid antibody, as well as its titer, is of diagnostic value in patients with serious infections caused by Staphylococcus aureus. The detection and quantification of teichoic acid antibodies is of great value for the early diagnosis of patients with acute osteomyelitis caused by Staphylococcus aureus and for assessing the clinical response of such patients.