The effect of leukocyte hydrolytic enzymes on periodontopathic bacteria was examined in vitro. A frozen and thawed extract of human peripheral blood leukocytes (LE) and human gingival crevicular exudate (GE) were shown to be able to cause the release of 50% of the radioactivity from a leukotoxic strain of Actinobacillus actinomycetemcomitans (Aa Y4), labeled by 14C. A nonleukotoxic strain (Aa 653) was shown to be more susceptible to both LE and GE, up to 68% of the total radioactivity was solubilized by LE at pH 7.4. Both bacterial strains were found to be resistant to the activity of lysozyme, but highly susceptible to lysolecithin and mixtures of lysolecithin and lysozyme or LE. Capnocytophaga sputigena strain 4 was also found to be partially susceptible to the effect of LE and GE. The possible role of leukocyte hydrolytic enzymes in bacteriolysis and release of bacterial products in relation to periodontal disease is discussed.