The particle size distribution of the quartz (14808607) fraction of lung dust and respirable mineral dusts was examined. Dusts extracted from the lungs of 26 deceased workers who had been employed in copper and uranium mines and tunnels were analyzed for quartz. The mass distributions of the total dusts and quartz fraction of the dusts were determined. A similar analysis was performed on respirable dusts obtained from lead and zinc mines, a dina factory, and foundry. The total mass of the dusts extracted from the lungs of the deceased workers ranged from 355 to 24024 milligrams (mg). The mass median unit density sphere diameter (MMD) of the dusts ranged from 2.4 to 8.1 microns. The percentage of quartz in the dusts ranged from 5.7 to 43.8%. On a mass basis, the quartz content of the dusts ranged from 51 to 4276mg. All subjects except eight whose lungs contained less than 235mg quartz had evidence of silicosis. The MMDs of the quartz component of the lung dusts ranged from 3.2 to 10.5 microns. The MMDs of the quartz fraction of the lung dust of 19 subjects were larger than those of the corresponding total lung dust. The MMD of the quartz component in the other subjects was close to that of the total dust. The quartz content of the dusts collected in the mines, dina factory, and foundry was 9.6, 65.4, and 5.3%, respectively. The MMDs of the total dust were 5.9, 5.3, and 6.2 microns, respectively. The MMDs of the quartz fraction were 6.0, 5.5, and 6.6 microns, respectively. The authors conclude that determining the particle sizes of the quartz fraction of mixed dusts may be helpful in characterizing the properties of the dusts.