A dual immunoassay of human chorionic gonadotropin (HCG) and human placental lactogen (HPL) at a microfabricated substrate was studied using scanning electrochemical microscopy (SECM). Anti-HCG and anti-HPL were microspotted on 50 X 50 mu m(2) raised flat squares separated from each other by 150 mu m at a glass substrate. A 10 mu l sample solution containing HCG and/or HPL was then dropped onto the substrate followed by treatment with a solution of horseradish peroxidase (HRP)-labeled anti-HCG and HRP-labeled anti-HPL The SECM measurements were carried out at 0.05 V vs. Ag/AgCl in the presence of ferrocenylmethanol (FMA) and H2O2 to detect FMA(+) generated by the HRP reaction. The SECM images of the substrates showed clearly the large response areas which corresponded to the squares with HCG and/or HPL. The detection limits were 0.1 IU ml(-1) for HCG and 3 ng ml(-1) for HPL.