A commercial kit from Diagnostica, Inc., Miami Fla., was studied for its ability to detect antibodies to the teichoic acids of Staphylococcus aureus. A comparative study of the Diagnostica counterimmunoelectrophoresis (CIE) system and our gel double-diffusion method was undertaken with 156 serum samples from 142 patients. Included were 25 cases of staphylococcal and non-staphylococcal endocarditis, 30 cases of S. aureus bacteremia, 19 cases of nonbacteremic S. aureus infection, 39 cases of hospitalized patients without a staphylococcal infection, and 29 normal controls. Agreement between methodologies was attained in 138 (88.5%) of the 156 samples tested and in 127 (89.4%) of the 142 patients. Of 13 patients with culture-proven S. aureus endocarditis, significant antibody titers were found in all patients (100%) by CIE and in 12 patients (92.3%) by double diffusion. No significant titers were found in normal sera by CIE, but four sera were positive by double diffusion. Of 80 sera from patients with no evidence of S. aureus infection, 4 (5.0%) were positive by CIE and 7 (8.8%) were positive by double diffusion. The Diagnostica CIE kit appears to provide a suitable means for the detection of deep-seated S. aureus infections.