Sera from 83 patients with campylobacter gastroenteritis were examined for the presence of legionella antibodies by indirect immunofluorescence. Twenty-one patients (25%) had positive titres (> or = 16) including 11 patients with titres of > or = 128. Legionella seropositivity persisted in 5 of 9 patients (55%) studied for 6-9 months. Campylobacter isolates were serotyped by the Penner scheme. Isolates associated with legionella seropositivity included Penner types 1, 2 and 4, the common endemic serotypes in England. Campylobacter blocking fluids were prepared from a range of Penner reference strains. The blocking fluid prepared from Penner type 11 was the most efficient at inhibiting the false-positive legionella titres. Using this absorption step legionella titres were inhibited from 24 of 26 patients (92%) with campylobacter but not from 8 patients with culture-proven legionnaires' disease. We recommend that this method is incorporated into routine diagnostic legionella serology in order to eliminate false-positive reactions due to campylobacter.