Using a newly developed catheter with a mobile, controllable tip, bronchial brush biopsies were performed in 50 consecutive patients with mid- and peripheral lung lesions suggestive of malignancy. Prior bronchoscopic examination of all patients had yielded no visual or cytologic evidence of malignancy. Of the 50 patients, 31 were subsequently shown to have malignant lesions. The cytologic results were diagnostic or highly suggestive in 25 (81 per cent) of the 31 patients, which is 10 per cent greater than the figures reported using premolded, fixed, polyethylene catheters. The 15 patients with definitely malignant cells all had malignant tumors. Complications were negligible. One mild pneumothorax occurred, but the patient required no therapy. It was concluded that the selector catheter system was rapid, safe, and accurate. Access to peripheral lung lesions was greater than that achieved by a nonmobile system.