To identify the immunoglobulin classes present in dental periapical lesions and to determine how, if at all, the distribution of immunoglobulin classes IgG. IgM and IgA differ in the dental periapical lesions of rheumatoid and control patients, paraffin-embedded and frozen tissues from the dental periapical lesions of 36 rheumatoid and 22 control patients were examined by the direct immunofluorescence technique. In addition, both rheumatoid and control tissues were examined for the presence of complement component C3, albumin and fibrinogen. Neither the greater frequency and abundancy with which IgG and to a lesser degree, IgA occurred in the dental periapical lesions of rheumatoid patients than in the corresponding lesions of control patients, nor the more frequent occurrence of complement component C3 in these same lesions, was statistically significant. No difference in the distribution of the various immunoglobulin classed could be discerned between the rheumatoid and control group. Albumin was distributed almost equally between the two patient groups. The greater abundance of fibrinogen among control patients was, however, statistically significant. These results do not provide evidence that an immunologic reaction takes place in the dental periapical lesions of patients with rheumatoid disease similar to that which characterizes rheumatoid inflammation. However, the presence of such a reaction cannot be excluded without more extensive investigation.