Despite their diverse application potential, carbon nanotubes (CNT) have adverse effects in vitro, and in vivo. Previous research has focused on the in vitro cytotoxic impact of CNT aggregates and associated nanoparticulate impurities. In this study, we compared the single-walled carbon nanotube (SWNT) aggregates, and their associated finely dispersed, non-aggregated carbon nanomaterials on rat aortic smooth muscle cells (SMC), through filtration of the aggregates from the CNT-treated cell culture media. In general, our research shows that the removal of single-walled carbon nanotube (SWNT) aggregates from cell culture test media inhibited the growth in SMC to a lower extent than the corresponding unfiltered media at pre-filtered SWNT dosages below 0.10 mg/ml. We also found suspended nanoparticles (likely amorphous and graphitic carbon associated with the SWNT) and a small quantity of SWNT in the filtered media may have contributed to the observed cell growth inhibition by the filtered media. In addition, we compared the effect of SWNT, a nano-sized material, with activated carbon (AC), a nanoporous, microparticulate material, on SMC growth. AC (0.1 mg/ml) was found to be less inhibitory to SMC growth than the SWNT aggregates and suspended matter (0.1 mg/ml), potentially implying an inverse proportionality between carbon nanomaterial size regimes and cell growth inhibition.