Factors contributing to the binding and reversible inactivation of gentamicin by purulent exudates were studied in a simplified in vitro model consisting of purified human polymorphonuclear leukocytes (PMNLs). Whereas intact PMNLs (10(6)-10(8)/ml) bound almost no [14C]gentamicin, freeze-thawed PMNLs showed extensive [14C]gentamicin binding, expressed as antibiotic cosedimenting with particulate material from the lysed PMNLs. Antibiotic binding could be related to the concentration of lysed PMNLs and to the amount of [14C]gentamicin added. Binding of [14C]gentamicin by lysed PMNLs was highly sensitive to DNase I but was unaffected by RNase, Triton X-100, or protease. Purified chromatin or DNA from either purulent exudates or lysed PMNLs reproduced the [14C]gentamicin-binding pattern obtained with crude PMNL lysate. These results show that gentamicin inactivation in purulent exudates can be correlated with binding of the antibiotic to lysed PMNLs; PMNL chromatin DNA is identified as one of the major binding factors.