Cell material from three different dental plaque bacteria (Actinomyces viscosus, Streptococcus mitis, Actinobacillus actinomycetemcomitans) were studied for their capacity to induce leukocyte migration in the dental pulp of monkeys. Altogether five animals were used to provide 123 teeth for the study. The bacterial test materials were prepared from lyophilized sonicates of pure cultures of respective bacteria mixed with crystalline bovine serum albumin (BSA) 1:1 by weight. Immediately prior to use in the experiment saline was added to render the test material into a paste-like consistency. Buccal Class V cavities were prepared in the monkey teeth leaving a thin remaining wall to the pulp. In these cavities, the test materials were enclosed for a period of 8 h. A set of control cavities received BSA alone. Placement of test and control materials were made before and after depleting the animals of serum complement by repeated injections of Cobra Venom Factor. Histologic examination of pulp tissue specimens showed that materials from two of the bacterial species (A. viscosus, Act. actinomycetemcomitans) consistently induced infiltrations of polymorphonuclear leukocytes in one to several cell layers underneath the test cavity. The material from S. mitis seemed less potent and in 11 of 22 teeth no pulpal reaction was recorded. Inactivation of complement with Cobra Venom Factor did not appear to affect the severity of the pulpal responses.