Teichoic acid antibodies (TAA), measured by gel diffusion, and antistaphylolysin (ASa) values were determined in 46 osteomyelitis patients and 200 healthy controls. A positive TAA titer (greater than or equal to 4) was seen in 4/16 patients with acute hematogenous osteomyelitis caused by Staphylococcus aureus and in 7/22 patients with chronic staphylococcal osteomyelitis, while ASta was positive (greater than or equal to 2.0 IU/ml) in 9 patients in both groups all except 2 exceeding 4.0 IU/ml. No positive TAA titres or ASta values were seen in 8 patients with chronic nonstaphylococcal osteomyelitis. The rate of positive test results in 200 controls was 7% for TAA and 4% for ASta. Basing diagnosis on a positive value in either of the tests gave 21 positive results in 38 patients with staphylococcal osteomyelitis; 18 by ASta alone and 11 by TAA alone. Both tests were positive at the same time in 3 patients with acute and in 5 patients with chronic staphylococcal osteomyelitis but never in controls.