Approximately 4 weeks after patients in ACTG 021 and ACTG 081 have begun PEN aerosol therapy for the prevention of PCP, patients undergo the first radioactive aerosol study. The patient inhales a mist while sitting with his/her back against a gamma camera. The resulting picture outlines the lungs. This mist contains a single dose of PEN mixed with a small amount of radioactivity (99m-technetium ertechnetate). The gamma camera determines where the particles deposit in the lungs. The radioactivity exposure is equivalent to a typical xray of the ribs. This procedure takes about 45 minutes; 6 hours later another 30-minute gamma camera image is obtained. Blood is withdrawn to measure the blood level of PEN as done in ACTG 021 and ACTG 081. The entire procedure is repeated near the end of the prophylactic schedule (about 10-12 months after the start of aerosol PEN treatments).