Recent studies have reported elevated levels of diesel particulate matter inside school bus passenger compartments. The elevated particulate levels have been attributed to the self-pollution from the school bus tailpipe and/or crankcase. Up to date there are still uncertainties as to the source of the particulate matter found in-cabin which can be attributed to the bus self-pollution as well as ambient pollution, neighboring vehicles, and re-entrained road dust. The issue has gained national attention because children are a particularly susceptible group to the adverse health effects from diesel particulate matter. The objectives of this study are to measure the concentrations of fine and ultrafine particles within the cabin of a school bus with and without retrofit technologies. To satisfy these objectives, mobile tests were conducted with a single school bus powered by an International DT466E engine on an outdoor test track at the Aberdeen Test Center in Aberdeen, MD. The study compared the ambient with the in-cabin pollutant levels measured during the operation of the bus following a realistic and repeatable test cycle developed using GPS data from New Jersey school bus routes. Particulate matter concentrations were measured using three Thermo Electron DataRAM-4 units, and three TSI P Trak ultrafine particle counters. Gaseous emissions (CO, CO2, HC, NOx) as well as pertinent engine parameters such as engine speed, fuel flow rate, engine oil temperature, and percent engine load were measured using the Sensors SEMTECH-D tailpipe emissions analyzer. The retrofit technologies used were a crankcase ventilation system for the crankcase emissions; a flow through filter, and a diesel particulate filter for tailpipe emissions control. Testing results indicate that operating the bus with the windows open results in only low levels of particulate matter (PM) during the cycle, but when operating with the windows closed a significant accumulation is observed. The combination of a closed crankcase filtration system with a tailpipe retrofit significantly improves the air quality within the cabin. This study suggests that current retrofit technologies can nearly eliminate diesel particulates from self-pollution inside school bus cabins.