There are no generally accepted diagnostic tests for cat-scratch disease (CSD), the cause of which is unknown. During the development of an indirect fluorescence antibody (IFA) test for detection of antibodies to "Rochalimaea henselae", sera from patients with CSD were found to have high titres to R henselae antigens. Further tests with this assay showed that 36 (88%) of 41 patients with suspected CSD had serum titres of 64 or more to R henselae antigen, that there was a low prevalence (3%) of substantial titres to R henselae in healthy controls (n = 107), and that the immune responses to R henselae antigens were specific. These data suggest that the R henselae IFA test, or other serological assays based on R henselae, may be useful for diagnosis of CSD.