Carbon nanotubes (CNT), despite their diverse application potential, have demonstrated adverse impacts in vitro and in vivo. Previous studies have focused on the combined in vitro cytotoxic impact of CNT aggregates and associated nanoparticulate impurities. However, the isolated effect of CNT aggregates and associated non-aggregated nanoparticulates have not been addressed in detail. In this work, the impact of single-walled nanotubes (SWNT) on rat aortic smooth muscle cells (SMC) was examined for SWNT (0.0-0.1 mg/ml) over a 3.5-day time-course. Cell culture medium was filtered to remove the aggregate material and both nanomaterial (un-filtered) and filtered SWNT media were used to examine cell growth. In general, the removal of SWNT aggregates from cell culture test medium by filtration increased the SMC number in comparison to unfiltered medium at pre-filtered SWNT dosages below 0.1 mg/ml. However, at 0.1 mg/ml, both filtered and unfiltered media exhibited a similar decrease in cell number relative to the control medium. The filtered medium was characterized and contained both suspended nanoparticles as well as a small quantity of SWNT, which may have contributed to the observed cell growth inhibition. As a comparison to the SWNT, activated carbon (0.1 mg/ml), a nanoporous, microparticulate carbon material, was found to be less inhibitory to SMC growth than the SWNT at the same dosage, implying an inverse proportionality between carbon nanomaterial size regimes and cell growth inhibition.