There is widespread interest in the use of interfacial layers in the preparation of diamond and diamond-like carbon (DLC) coatings. These DLC coatings deposited onto metal substrates with a silicon carbide (SiC) interfacial layer exhibit improved adhesion and show promise as wear-retardant coatings for biomedical implants. Although the DLC coatings show excellent biocompatibility in vitro, they may be susceptible to damage within the biological environment, leading to exposure of the interfacial silicon carbide. We have investigated the effects of two forms of silicon carbide (alpha-SiC and beta-SiC) on macrophages, fibroblasts and bone cells in vitro. Both alpha- and beta-SiC are well-tolerated by cells at concentrations of up to 0.1 mg/ml but cause severe cytotoxicity at a concentration of 1 mg/ml. If SiC is to be used as an interfacial layer on biomedical implants, the quality of the DLC coating must be optimised to minimise the risk of film breakdown.