The Code of Federal Regulations defines specific size and silica limitations for rock dust that is used in the underground coal mining industry. MSHA collected 444 grab samples of rock dust from mines located in Districts 2 through 11 and made these samples available to NIOSH for analysis. XRF and XRD analyses were completed on 261 rock dust samples, representing 23 producers and seven distributors. Results from the XRF analysis show that 93.5% of the analyzed samples meet the CFR requirement of containing 4% or less of FCS. Another 5.7% of the samples contained between 4.1 and 5% FCS, which can be approved for use by the Secretary of Labor. Consequently, only 0.8% of the samples analyzed contained FCS above the maximum allowable limit. Fifteen or more samples of rock dust were analyzed for six different producers. These samples were delivered to the mines in 40- or 50-lb bags or in bulk quantities. Comparison of the percentage of FCS and quartz content for the bagged and bulk products showed that these percentages were within 0.1% of each other for eight of 12 comparisons, with a maximum difference of 0.5% for FCS and 0.4% for quartz in the four remaining samples. Although not specified in the CFR, NIOSH was interested in quantifying the quartz content in the rock dust samples and had XRD analysis completed on these grab samples. Even though this analysis was not specifically conducted on the respirable fraction of the grab samples, the presence of rock dust continued quartz and a high respirable content would provide the potential for quartz inhalation by mine workers when exposed to airborne rock dust. A summary of these results showed that 97.3% of the samples contained less than 2% quartz. This quartz content is relatively low when compared to the 5% quartz level that must be exceeded in airborne respirable coal mine dust samples to result in a reduced respirable dust standard. However, size analysis of the rock dust samples showed that 87.7% of the samples contained over 20% respirable-sized dust, while 29.6% of the samples contained over 30% respirable dust. Not all of the respirable-sized dust in these samples will be present as airborne respirable dust when the rock dust is applied in mine entries. Nonetheless, mine operators should be aware of the potential exposure to respirable rock dust containing quartz and take steps to minimize or eliminate the time that mine workers are exposed to airborne rock dust. For workers required to apply the rock dust, it may also be appropriate for them to wear respiratory protection while completing this task.