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Separate samples of supragingival dental plaque overtly free of blood were centrifuged to obtain the free fluid phase (plaque fluid). Bound protein was eluted from the plaque bacteria and matrix by washing the plaque with a low-pH buffer. The plaque fluid, low pH eluate, and whole saliva were assayed for immunoglobulins A, G, and M, the third component of complement, lysozyme, lactoferrin, and lactoperoxidase. Concentrations of total protein and albumin were also determined. Antibody reactive with specific plaque bacteria was detected by indirect immunofluorescent microscopy. Specific and nonspecific immune proteins were present in plaque fluid from adult subjects at significantly greater concentrations than in their saliva, which suggests that these proteins are concentrated in dental plaque. The results indicate that both saliva and gingival exudate contribute to the immunological proteins found in the free fluid phase of dental plaque. The observation that immunoglobulin A antibody reactive with plaque bacteria could be detected in plaque fluid suggests that a wide variety of immunological reactions may occur in the dental plaque. These potential interactions between host, plaque bacteria, and their products could serve to influence the plaque flora and its ability to induce disease.