A technique for the characterization of rat gingival lymphocytes has been described. The technique was used to obtain gingival cells from rats maintained on antigen-free diets or such diets with ovalbumin (OVA) added. Increases in gingival lymphocyte numbers in the antigen-fed (AF) animals occurred by 16 to 23 days of OVA feeding. The elevated gingival lymphocyte numbers were predominantly T lymphocytes at the initial intervals of the experiment (to 59 days of OVA feeding). At 128 days of OVA feeding T-lymphocyte numbers diminished but B-lymphocyte numbers increased, and AF animals had more than six times as many gingival B lymphocytes as animals not fed antigen. Also, AF animals showed immunoglobulin A antibody in intestinal perfusates (after 9 days of OVA feeding) and in saliva (within 23 days of OVA initiation). Plasma immunoglobulin G antibodies were not detected until 59 days of feeding. Spleen cells from AF rats showed in vitro blastogenic responses to OVA at 23 to 59 days of feeding. Periodontal bone loss was greater in AF animals after 59 and 128 days of OVA. Germfree animals fed only one antigen experienced more periodontal bone loss than animals fed the same diet not containing antigen. Therefore, immune phenomena can contribute to experimental bone loss in germfree rats.