The purpose of this investigation was to study inflammatory cell populations apical to the buccal and interproximal gingival sulcus during destructive experimental marginal periodontitis in squirrel monkeys. Specimens were obtained of the clinically healthy periodontium, and (after 2 and 10 weeks) of experimental periodontitis induced by the placement of plaque-retentive silk ligatures at the gingival margin. Cell populations were characterized and quantitated in a standard area of connective tissue immediately subjacent to the most apical cells of the junctional epithelium in buccal and interproximal locations. Comparisons of periodontitis cell populations showed there were no differences between the regions in relation to either total numbers of cells, or in the percentages of inflammatory cell types. The inflammatory cells consisted primarily of granulocytes and macrophages, with only small percentages of lymphoid and plasma cells. Mechanisms and consequences of these cell types relative to periodontal tissue destruction are discussed.