As regulatory limits in the US for 2007 heavy-duty diesel engines introduce dramatic reductions in PM emissions, there is considerable interest in new emission metrology that can more accurately measure low PM levels. One such metrology, particle number measurement, has been extensively investigated in Europe as part of Europe's Particle Measurement Programme (PMP). This program has put forth a new methodology, including instrument specifications and sampling protocols, for "solid" particle number measurements. While counting only solid particles results in better precision, it may not be fully indicative of the diesel PM exhaust components of interest from a health effects perspective. The PMP protocol still represents a significant advancement as it is currently the only methodology with low enough detection limits to produce precise measurements of DPF equipped engines. The specific objective of this study was to critically evaluate the proposed PMP method for determining "solid" particle number emissions from in-use heavy-duty vehicles during "real-world" over-the-road driving. For this program, CARB sought to conduct testing over the road with the CECERT mobile emissions laboratory (MEL). PMP compliant dilution systems for measuring solid particle number were tested along with a full suite of other particle measurements and compared directly with filter-based PM measurements on a heavy-duty truck equipped with a DPF. Gravimetric PM filter mass measurements were near detection limits for most of driving cycle, which makes it desirable to explore new PM measurement protocol. The particle number measurements using PMP method showed a lower coefficient of variation than the PM filter mass measurements.