This study investigated the cytotoxic potential of novel activated carbon adsorbents (MAST Carbon International Ltd.) developed for medical applications such as extracorporeal therapies. Carbon adsorbents were assessed for their in vitro cytotoxicity against a V79 cell line using a material extraction method in combination with a colony formation assay. Results were compared to those from a commercially available cellulose-coated carbon adsorbent, developed for direct haemoperfusion. Initial findings demonstrated an inhibition of colony formation and an apparent cytotoxic effect. However, it was found that this inhibition occurred as a result of protein and ion adsorption by carbon materials possessing large surface area and highly developed porous structure. Consequently, these essential nutrients were unavailable to the cells during colony formation. Modifications to the cytotoxicity assessment methods were required in order to take into account nutrient loss. Subsequently it was determined that the carbon materials do not show a cytotoxic response towards the V79 cell line under the modified conditions employed. The suggested approach may be useful in the assessment of other biomaterials such as carbon nanotubes and other nanoparticles which possess large surface area. The preliminary data support the ongoing investigation of these adsorbents as candidates for use in extracorporeal therapies.