The design, fabrication and evaluation of chemically modified screen-printed carbon electrodes is described with particular emphasis being placed on the practical details for sensor construction. This work employed cyclic and differential-pulse voltammetry, together with amperometry in stirred solutions to investigate systematically electrodes containing phthalocyanine and ferrocene-based mediators for the determination of several important biomolecules. An overview of the development of an amperometric assay using the enzyme glutathione peroxidase, and some preliminary results, for the selective determination of reduced glutathione in human whole blood is also presented.