the air at these cities was determined by use of RAC gas samplers with two sampling tubes in each box and a common membrane prefilter for both sampling tubes. The particulate lead collected on the membrane prefilters was not determined for this study. Two replicate organic lead samples were collected in each 24-hour sampling period at each of the six cities. The organic lead analyses for each city (Table IV) indicate that the average level of organic lead at these cities is about 0.2 Aig/m3 4. Although this value is close to the detection limit of the method, it appears to be relatively constant for the six cities sampled. Five of the samples listed in Table IV showed organic lead values greater than 0.5 ¿ig/m3; however, two replicate values for each of these samples almost invariably did not agree, an indication of error in the collection or determination of these samples. This number of questionable samples was not unexpected, considering that this study was the first field evaluation of the method. Samples for which replicate values differed from their average by more than twice the standard deviation (34% relative standard de-viation at 0.4 Mg/m3) were not included in calculations of the average organic lead value for each city.In order to determine the total airborne lead and percentage lead in the particulate form, organic lead was sampled with bubbler boxes side-by-side and concomitantly with collection of suspended particulate matter on glass fiber filters using a Hi-Vol sampler. The results of this study are given in Table V; although almost all (89%) of the lead found in the atmosphere in this study was in the form of partícula te matter. Although the organic lead levels were found to be comparable in all cities, the particulate lead concentrations varied by as much as a factor of 3 from city to city. This suggests that organic lead in urban air approaches a steady state.