These studies concern the biological properties of theActinobacillus actinomycetemcomitansleukotoxin in different primates. We compared the sensitivity of primate polymorphonuclear leukocytes (PMNs) to the leukotoxin and determined whether animals had toxin‐neutralizing antibodies in serum. In addition, subgingival plaques from representative non‐human primates were analysed for leukotoxin‐producingA. actinomycetemcomitans.PMNs from different primates varied significantly in sensitivity to leukotoxic extracts of A. actinomycetemcomitans. Neutrophils from humans, the Great Apes and most Old World Monkeys were destroyed uponin vitroexposure to the toxin. By contrast, PMNs from Prosimii, New World Monkeys and the Lesser Apes were not sensitive to the leukotoxin. Human PMNs were more sensitive when compared to PMNs of non‐human primates. Chimpanzee PMNs were closest to human PMNs in terms of sensitivity to the toxin. The data indicate that the leukotoxin has unique specificites for primate PMNs and is especially potent against human leukocytes. Sera from the vast majority of adult primates were capable of neutralizing theA. actinomycetemcomitansleukotoxin, including animals whose PMNs were resistant to the toxin. This suggests that most non‐human primates have been exposed to toxin‐producing strains ofA. actinomycetemcomitans, and this was confirmed in representative species where leukotoxicA. actinomycetemcomitanscould be isolated from subgingival plaque samples. By contrast, less than 20 of sera from normal humans neutralized the leukotoxin whereas over 90 of sera from juvenile periodontitis patients had relatively high toxin‐neutralizing antibody liters. It appears that the majority of adult primates harbor A. actinomycetemcomitans and produce antibodies against the leukotoxin of this organism. In humans, on the other hand, these phenomena are usually associated with destructive periodontal disease. Differences between man and non‐human primates in terms of their sensitivity to and/ or in the dynamics of the immune response against the leukotoxin, may be important determinants in explaining why this organism seems to belong to the normal indigenous oral microflora in some primates but acts as a periodontal
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