To determine the usefulness of the teichoic acid antibody (TAA) test in conditions where unspecific viral and bacterial antibodies are often encountered, we measured TAA by the gel-diffusion method in 475 patients without known staphylococcal disease; they included 213 patients with arthritis, 108 with liver diseases, 100 with gastro-intestinal disorders and 54 with acute pharyngitis. Positive controls were 104 patients with Staphylococcus aureus bacteraemia and 203 healthy adults were negative controls. Thirteen (6%) of the healthy adults had positive TAA titres (greater than or equal to 4), and the highest titre was 8 in two people (1%). Positive titres were found in 38% of patients with S. aureus bacteraemia and high titres (greater than or equal to 8) were seen in 24%. Among the patients with arthritis, positive TAA titres were found significantly more often than in healthy controls in patients with Yersinia arthritis (p less than 0.01) and systemic lupus erythematosus (SLE; p less than 0.02). In other patient groups, the percentage of positive TAA titres did not differ significantly from that in healthy adults. Eight (2%) of the 475 patients without known staphylococcal infection had TAA titres greater than or equal to 8 but these high titres were not associated with any particular disease group. Only two of these eight patients had slightly raised antibody to staphylococcal alpha-haemolysin. We conclude that the TAA test cannot be used as a reliable indicator of septic staphylococcal disease in patients with Yersinia arthritis or SLE, but that in general, TAA titres greater than or equal to 8 point strongly to S. aureus infection even in patients with autoimmune or liver diseases.