Actinomyces are Gram-positive, non-acid-fast, anaerobic or microaerophilic filamentous bacteria. These organisms are frequently detected from infected root canals and periapical lesion. The purpose of this study was to use indirect immunofluorescence to determine the prescence of select Actinomyces species in a survey of teeth associated with periapical lesion, to clarify the relationship between clinical symptoms of periapical lesions and the Actinomyces species and to study on the cross reaction among Actinomyces. Actinomyces israelii serotype I (ATCC 12102), Actinomyces israelii serotype II (ATCC 29322), Actinomyces viscosus serotype II (ATCC 19246), Actinomyces naslundii serotype I (ATCC 12104) were cultured in anaerobic condition. Rabbit antisera were prepared by intravenous injection of formalized whole cells. Indirect immunofluorescence method was used to achieve the purpose. The following results were obtained. 1. There was a relationship between Actinomyces and periapical disease. 2. A. israelii serotype I, II were frequently identified with Indirect Immunofluorescence and most often assosiated with periapical disease. In culture finding, there was no significant difference between each group. 3. Indirect Immunofluoresence is both more sensitive and more rapid than culture for identification of Actinomyces species in patients with periapical lesion. 4. A. israelii serotype I, II was highly isolated in infected root canals with local swelling, A. naslundii serotype I was highly isolated in those with foul odor, and A. israelii serotype I was found in higher frequncy in those with exudate than other bacteria. 5. In the Indirect Immunofluorescence (1 : 320), A positive cross reaction was obtained between A. israelii serotype I and A. israelii serotype II, also, A. viscosus serotype II and A. naslundii serotype I. There was no cross reaction between A. israelii serotype I, II and A. viscosus serotype II, A. naslundii serotype I.