Bacterial respiratory infections are common in patients undergoing prolonged mechanical ventilation, and antibiotic selection often is based upon the results of smears and cultures of tracheal aspirates (TA). This study was designed to determine the reliability of gram stains, cultures and antibody-coating of bacteria in TA by comparing them with the results of quantitative cultures of specimens obtained by protected brush catheters (PBC) inserted into involved areas of lung parenchyma. Twenty-two patients on mechanical ventilation for at least 72 hours, with new radiographic infiltrates and fever, were studied. Tracheal aspiration was performed in the usual manner using sterile disposable kits. Immediately thereafter, patients underwent fiberoptic bronchoscopy, and PBC-specimens were obtained from the areas of new radiographic infiltrates. Tracheal aspirates were gram stained and cultured aerobically, and antibody coating of bacteria was determined by fluorescence microscopy. Quantitative culture of PBC specimens contained greater than or equal to 10(3) colony forming units per brush in 16 of the 22 patients; a smaller number of organisms was present in two additional patients. In TA from all 22 patients, gram stains revealed polymorphonuclear neutrophils and bacteria. Cultures of TA revealed potential pathogens in 20 patients, and in 15, multiple pathogens were present. The bacteria isolated from PBC also were present in 14 of the 16 patients with greater than or equal to 10(3) CFU in PBC cultures (88%). Antibody coating was present in TA from 12 patients, and antibody coating correlated poorly with cultures of PBC specimens.(ABSTRACT TRUNCATED AT 250 WORDS)