Purified carbon nanotubes are new carbon allotropes, sharing similarities with graphite,that have recently been proposed for their potential use with biological systems as probesfor in vitro research and for diagnostic and clinical purposes. However the biocompatibilityof carbon nanotubes with cells represents an important problem that, so far, remainslargely uninvestigated.The objective of this in vitro study is to explore the cytocompatibility properties of purifiedcarbon nanofibres with cardiomyocytes.Cardiac muscle cells from a rat heart cell line H9c2 (2-1) have been used. Highlypurified single-walled nanotubes (SWNTs) were suspended at the concentration of0.2 mg ml−1 by ultrasound in complete Dulbecco’s modified Eagle’s medium, and administered to cellsto evaluate cell proliferation and shape changes by light microscopy, cell viability by trypanblue exclusion, and apoptosis, determined flow cytometrically by annexin/PI staining.Microscopic observation evidenced that carbon nanotubes bind to the cell membrane,causing a slight modification in cell shape and in cell count only after three days oftreatment. Cell viability was not affected by carbon nanotubes in the first three days ofculture, while after this time, cell death was slightly higher in nanotube-treated cells(p = ns). Accordingly, nanotube treatment induced little and non-significant change in theapoptotic cell number at day 1 and 3.The effect of nanotubes bound to cells was tested by reseeding treated cardiomyocytes.Cells from a trypsinized nanotube-treated sample showed a limited abilityto proliferate, and a definite difference in shape, with a high degree of celldeath: compared to reseeded untreated ones, in SWNT-treated samples theannexin-positive/PI-negative cells increased from 2.9% to 9.3% in SWNT(p<0.05,where p<0.05 defines a statistically significant difference with a probability above 95%),and the annexin-positive/PI-positive cells increased from 5.2% to 18.7%(p<0.05). However, overtime cells from a trypsinized nanotube-treated sample continued to grow,and partially recovered the original shape.In conclusion our results demonstrate that highly purified carbon nanotubes possess noevident short-term toxicity and can be considered biocompatible with cardiomyocytes inculture, while the long-term negative effects, that are evidenced after reseeding, areprobably due to physical rather than chemical interactions.