Human dental plaque extracts in Gey's medium and whole human saliva were tested in the Boyden Chamber system for chemotactic activity against rabbit peritoneal polymorphonuclear leukocytes. Positive controls were a semipurified bacterial chemotactic factor and a complement derived chemotactic factor, Gey's medium alone served as a negative control. The method appeared to be very sensitive as concentrations of less than one mg plaque per ml medium gave a clear chemotactic response. Whole human saliva was less chemotactic than dental plaque. Incubation of equal concentrations of plaque extract in both the upper and lower compartments of the Boyden Chamber did not result in a large increase in cell migration, indicating that most cell migration in reaction to dental plaque extract in this system depends on a concentration gradient. The chemotactic factor or factors were present in plaque at the time it was collected, they were readily dissolvable in water, and they were heat stable. The dose range in which dental plaque extract would evoke a chemotactic response was variable and usually very narrow. Replicate tests. yielded highly variable results indicating that this method is not suitable for quantitative comparisons. Extracts of the same dilution were prepared from dental plaque samples from a small population with varying degrees of gingivitis. Differences in chemotactic response to those samples could not be related to the intensity of gingivitis.