The field of nanotoxicology recently has emerged out of the need to systematically study the biocompatibility and potential adverse effects of novel nanomaterials. Carbon nanotubes (CNT) are one of the most interesting types of nanomaterials, and recently, their use in applications has dramatically increased. Their potential adverse impact on human health and the environment, however, have caused them to be viewed with apprehension in certain cases so further studies into their toxicology are justified. Current methodologies using cell culture (in vitro) models are unreliable and are not yet able to offer conclusive results about the toxicity profile of CNT. The need for reliable and rapid toxicity assays that will allow high throughput screening of nanotube materials is a prerequisite for the valid assessment of CNT toxicity. The assay described here was developed based on the pitfalls and drawbacks of traditionally used cytotoxicity assays. A methodological description of the main problems associated with the MTT and the LDH assays is offered to illustrate the advantages of this novel assay for the study and determination of the cytotoxic profile of CNT. Most importantly, a thorough account of this novel assay which is considered to be rapid, reliable, and suitable for broad-spectrum cytotoxicity screening of different types of CNT is described.