Carbon nanotube transistor arrays (CNTFETs) wereused as biosensors to detect DNA hybridization andto recognize two anabolic steroids, stanozolol (Stz)and methylboldenone (MB). Single strand DNA andantibodies specific for STz and MB were immobilizedon the carbon nanotubes (CNTs) in situ in the deviceusing two different approaches: direct noncovalentbonding of antibodies to the devices and covalentlytrough a polymer previously attached to theCNTFETs. A new approach to ensure specificadsorption of the biomolecules to the nanotubeswas developed. The polymer poly(methylmethacrylate0.8-co-poly (ethyleneglycol)methacrylate0.8-co-N-succinimidyl methacrylate0.1)was synthesized and bonded noncovalently to thenanotube. Aminated single-strand DNA or antibodiesspecific for Stz and MB were then attached covalentlyto the polymer. Statistically significant changes wereobserved in key transistor parameters for both DNAhybridization and steroids recognition. Regardingthe detection mechanism, in addition to chargetransfer, Schottky barrier, SB, modification, andscattering potential reported by other authors, anelectron/hole trapping mechanism leading tohysteresis modification has been determined. Thepresence of polymer seems to hinder the modulationof the electrode-CNT contact.