In the last few years several bacteriological and immunological studies have investigated the role of bacteria and immune defects in order to establish the etiopathogenesis of periodontal disease. With regard to the immune system, a defect in polymorphonuclear neutrophil (PMN) chemotaxis has been frequently reported in patients with rapidly progressive or juvenile periodontitis. The purpose of this study was to investigate in five patients with rapidly progressive periodontitis and normal chemotaxis of peripheral blood PMNs the presence of chemotaxis inhibitory activity in gingival fluid and to relate such activity to three types of bacteria, often involved in rapidly evolving periodontal lesions, that are able to inhibit in vitro PMN chemotaxis: Bacteroides gingivalis, Capnocytophaga sp., and Actinobacillus actinomycetemcomitans. We found strong inhibitory activity in three of these patients. This activity was consistently associated with the finding of B. gingivalis in gingival pockets. We cannot rule out, however, that other substances not of bacterial origin could be responsible for such inhibitory activity. The strict association with B. gingivalis, known to secrete blocking factors, is highly suggestive, although this data must be considered preliminary.