AIMS: To investigate the serological cross reaction between legionella and campylobacter using the rapid microagglutination test (RMAT). METHODS: Serum samples from 49 patients with campylobacter infection were tested for legionella antibodies using the indirect fluorescent antibody test (IFAT) and the RMAT. Serum samples that had positive RMAT titres were retested in the presence of a campylobacter immunosorbent. The specificity of the immunosorbent was evaluated with serum from patients with genuine legionella infection (legionella culture or antigen positive, or both). RESULTS: Fourteen (28%) patients with campylobacter infection had positive IFAT titres (> or = 16) and 16 (32%) patients had positive RMAT titres (> or = 8) in one or more serum samples. In addition, serum samples from 11 of 17 patients with campylobacter infection, previously shown to have positive legionella IFAT titres, were also RMAT positive. Sixteen patients had RMAT titres of > or = 32, including seven with titres of > or = 128. RMAT titres from all but one patient were significantly reduced after campylobacter absorption, but serum samples from 48 patients with legionella infection were unaffected. CONCLUSIONS: Serological cross reaction between campylobacter and legionella can occur in the legionella RMAT, as well as the IFAT. This cross reaction can be eliminated in most cases by incorporating a campylobacter immunosorbent in the RMAT.