Antibodies to Rickettsia conorii were detected by the indirect immunofluorescence (IFA) assay in 64 (51.6%) out of 124 examinees living in North Dalmatia (Croatia) who had a history of recent tick bites during 1994 and 1995. Positive titers of IFA antibodies to R. conorii were detected in 12 (31.5%) out of 38 examinees with carried out prophylaxis by azithromycin. The usual clinical signs of Mediterranean spotted fever were registered in 22 (25.6%) and asymptomatic infection in 30 (34.8%) out of 86 examinees without prophylaxis. Clinical signs of the disease were not registered in examinees with prophylaxis.