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In this paper we report a label-free biosensor able to detect 10 mg/L of human immunoglobulin G (HIgG) at physiological conditions. It is based on a field effect transistor in which a network of carbon nanotubes (CNTs) acts as the conductor channe l. HIgG an-tibodies are linked to the CNTs in three steps. First, the polymer polyethyleneimine (PEI) covers the CNTs’ surface preventing the non-specific binding of proteins. Second, the HIgG antibodies are linked to the CNTs using glutaraldehyde as a cross-linker. Finally, glycine is used to block the unreacted aldehyde groups and minimize unspecific adsorption effects. The selectivity of the sensor has been tested against 10 mg/L of serum albumin, the most abundant protein in plasma. Keywords: Carbon nanotube field effect transistor, human immunoglobulin G, CNT functionalization, immunosensor, antigen–antibody interaction, physiological conditions. INTRODUCTION Immunoanalytical techniques, such as enzyme-linked immu-nosorbent assay (ELISA), immunoelectrophoresis or fluoroimmu-noassays, based on the interaction between an antigen and the cor-responding antibody, have been used for many years due to their proven usefulness. However, the need to use labels and to add spe-cific reagents makes them frequently costly and time consuming. These techniques are not direct in the sense that they need a secon-dary mechanism to be able to detect the recognition event. There-fore, the development of new improved methods for the rapid de-tection of antigens or antibodies is important and applicable to sev-eral fields such as medical diagnosis, environmental analysis or forensic medicine. Immunoglobulin G is usually used as a model antigen when doing immunoassays [1]. Human Immunoglobulin G (HIgG) is one of the most important proteins found in human body fluids. Its primary function is related to defence mechanisms, when foreign agents have entered the human body. HIgG concentration changes are related to several diseases and, therefore, the protein is a relevant diagnosis indicator. CNTs were first seen by S. Iijima in 1991 [2]. Among other in-teresting characteristics, semiconducting CNTs modify their electri-cal conductivity when their nearest chemical environment changes [3]. In an attempt to take advantage of this property, CNTs have been integrated into field effect transistors (FETs) [4] as the semi-conducting channel of carbon nanotube field effect transistors (CNTFETs). CNTFETs have recently been used as biosensors and, nowadays, several kinds of biomolecules have been immobilized on CNTs: proteins [5-7], aptamers [8, 9], antibodies [5, 9], and DNA [10]. CNTFET based biosensors show several potential advantages such as needing a low volume of the sample, responding quickly, being sensitive, being label free and allowing miniaturization. Among the numerous studies on CNTs applied to biosensing, there are few reports which use a CNTFET and an antigen-antibody in-teraction to obtain an immunosensor. Takeda