This paper summarizes the Heavy-Duty In-Use Testing (HDUIT) measurement allowance program for Particulate Matter Portable Emissions Measurement Systems (PM-PEMS). The measurement allowance program was designed to determine the incremental error between PM measurements using the laboratory constant volume sampler (CVS) filter method and in-use testing with a PEMS. Two independent PM-PEMS that included the Sensors Portable Particulate Measuring Device (PPMD) and the Horiba Transient Particulate Matter (TRPM) were used in this program. An additional instrument that included the AVL Micro Soot Sensor (MSS) was used in conjunction with the Sensors PPMD to be considered a PM-PEMS. A series of steady state and transient tests were performed in a 40 CFR Part 1065 compliant engine dynamometer test cell using a 2007 on-highway heavy-duty diesel engine to quantify the accuracy and precision of the PEMS in comparison with the CVS filter-based method. Additional tests were performed to investigate the susceptibility of the PEMS to ambient pressure, ambient temperature, electromagnetic/radio frequency interference, and vibration. The experimental data were used as input to a Monte Carlo statistical model. The model was iterated to randomly combine the various sources of PEMS measurement errors, the results of which were used to determine the additive measurement allowances for in-use testing. This paper focuses on the laboratory efforts conducted to determine the PM-PEMS measurement allowance.It was discovered that the largest source of error was the ability to accurately measure the PM concentration level during steady state engine testing. The precision of the PEMS under transient operation contributed a much smaller but still significant error while the additional error observed during environmental testing was insignificant. The magnitude of the error surfaces was generally smallest for the AVL and largest for the Sensors PEMS. Validation of the model was performed with a mobile CVS laboratory during on-highway operation. The AVL PEMS validated for one of the three calculation methods, while the Sensors PEMS did not validate. Based upon successful validation of the AVL PEMS, the measurement allowance of 0.006 g/hp-hr, which was generated based on the Sensors PPMD laboratory efforts at an NTE threshold level of 0.02 g/hp-hr, was accepted for the HDUIT enforceable program. This measurement allowance value is added to the in-use threshold limit to take into account errors associated with PM-PEMS during in-use testing.