Carboxylic group functionalized single walled (SW) and multi walled (MW) carbon nanotubes (CNT) have been incorporated into biopolymer matrix of chitosan (CH) to fabricate nanobiocomposite film onto indiumtin-oxide (ITO) coated glass plate for co-immobilization of rabbit-immunoglobulin (r-IgGs) and bovine serum albumin (BSA) to detect ochratoxin-A (OTA). The results of electrochemical studies reveal that presence of both CNT results in increased electro-active surface area of CH leading to enhanced electron transport in these nanobiocomposites. Moreover, in CH-SWCNT and CH-MWCNT nanobiocomposites the availability of NH2/OH group in CH and surface charged CNT also increases loading of the r-IgGs resulting in enhanced electron transport responsible for improved sensing characteristics. Compared to BSA/r-IgGs/CH-MWCNT/ITO immunoelectrode, electrochemical response studies of BSA/r-IgGs/CH-SWCNT/ITO immunoelectrode carried out as a function of OTA concentration exhibits improved linearity as 0.25-6 ng/dL, detection limit as 0.25 ng/dL, response time as 25 s, and sensitivity as 21 mu A ng dL(-1) cm(-2) with the regression coefficient as 0.998. (C) 2010 Elsevier B.V. All rights reserved.