This report gives the results of three years' work on lead aerosols in the environment and the laboratory. The uptake of both radioactive and stable lead by human volunteers has been measured, with the object of estimating the uptake by humans of lead from petrol driven vehicles, and also the contribution of this to the total uptake of lead. Radioactive tracers were used to determine the effect of chemical composition of lead aerosols on uptake, and also to compare uptake from lung and gut. The effects of lead on health were not included in the work. Earlier work carried out in this project has been published and is reviewed. Exhaust aerosol labelled with 203 pb was inhaled by volunteers and the deposition in the lung and uptake by blood measured by means of the radioactive tracer. The results were used to calculate a value alpha for the relation between blood lead and air lead. A calculation based on 35% deposition of the inhaled aerosol in the lung, 50% transfer of the lead to red cells and a 16-day biological half-life in blood, gave a value of alpha of about unity. The work indicated the significance of particle size on deposition in the lung. (TRRL)