In this study the density of colloidal latex particles was determined to four figure accuracy by subjecting the colloid to a series of sedimentation field-flow fractionation runs in which the density of the aqueous carrier solution was systematically varied. Accuracy was gained by using high-field strengths which made possible the use of carriers with densities only 0.0005-0.05 g/cm(3) removed from the isopycnic particle density. Both positive and negative density differences were used, yielding independent measurements of particle densities that differed on average by only 0.0003 g/cm(3). At the high accuracy level achieved, a trend was discerned in which the apparent or isopycnic density of polystyrene latex particles increases systematically with size, gaining similar to 0.002 g/cm(3) over the diameter range 0.26-0.46 mu m. This trend is likely attributable to interfacial effects, particularly to an interfacial volume increment generated by structural rearrangements relative to the bulk phases at the particle-aqueous interface. A comparison of the data with theory yields, after correcting for Young-Lapace particle compression, a specific interfacial volume (volume increment per unit area of interface) of similar to 1.7 Angstrom and an ambient bulk polystyrene density of 1.0530 +/- 0.0003 g/cm(3). Suggestions are made for improving the work and extending it to the measurement of small second-order effects associated with adsorption, swelling, and polymer relaxation in the glassy state.