The introduction of accessible cysteine residues by genetic techniques is demonstrated to be a viable method to immobilize proteins onto gold surfaces in a controlled manner. DHFR-AS, a dihydrofolate reductase mutant which has had both native cysteine residues replaced, exhibited only limited absorption as determined by thickness-shear mode piezoelectric sensor and enzymatic assay measurements. With the addition of a cysteine residue to the C-terminal end of the enzyme (DHFR-AS-C), the level of adsorption was increased by a factor of approximately 4. The average of DHFR-AS-C over the gold surface is estimated to be 4 x 10(-12) mol/cm(2).