A simple process using technetium-99m generator eluate in a graphite crucible at 2500 degrees C, produces a structured ultra-fine dispersion of labelled carbon. Particle sizes are 5.0 nm (0.005 micron) and less and adhere to the walls of the alveoli on inhalation. Penetration characteristics are gas-like and the radioactivity per litre of carrier argon allows single breath inhalations of a diagnostic dose. Over 190 patients have been studied including 50 within a formal clinical trial with xenon-133. Results of tomography, dynamic inhalation, and image subtraction using this new agent-'Technegas'--are presented.