The chemiluminescent (CL) response induced by zymosan and polystyrene-butadiene particles in alveolar macrophages (AM) obtained from male New-Zealand-white-rabbits was reported. The CL magnitude was about four times greater with zymosan than with polystyrene-butadiene. The CL reached a peak at 15 minutes after introduction of the zymosan particles and was maintained for about 10 minutes. Light emission diminished gradually over the next 2.5 hours until at 3 hours it had almost disappeared. With polystyrene particles, the CL peaked in 10 minutes and disappeared within 90 minutes. The phenomenon did not result from particle uptake itself, but the amount of light produced by the cells did depend on the zymosan concentration until a certain saturation point was achieved. This maximum CL in the case of zymosan seemed to occur when there was one particle present in the medium for each cell. To test the role of cellular metabolism in the CL of alveolar macrophages, 0.5 millimolar 2,4-dinitrophenol (DNP) was added to the medium after maximal CL had been achieved. Within 5 minutes the CL response disappeared. When DNP was introduced 10 minutes prior to addition of the particles, no CL was observed. Incubation of the cells in a medium containing glycolytic inhibitors also eliminated the CL response. To determine if CL resulted from ATP production, various concentrations of ATP were added to the medium prior to the addition of zymosan particles. ATP caused no change in the temporal pattern of the response, but did affect the absolute level of light emission. As the ATP concentration in the medium increased, the maximal CL response decreased. This decrease may have resulted because the metabolic processes of the cell responsible for ATP production were inhibited as there was plenty of ATP available. The authors conclude that a metabolic origin of CL is likely.