The complement system is strongly activated by surfaces carrying nucleophilic groups, such as hydroxyl (OH) groups, and triggered by deposition of complement protein fragment, C3b. Surfaces carrying amino groups, the other representative nucleophilic group, are expected to be potential activators of the complement system through the alternative pathway. Few studies thus far have examined the potential of artificial materials carrying amino groups in activating the complement system. In this study, we employed a self-assembled monolayer (SAM) of 11-amino-1-undecanethiol (NH 2 -SAM) and a polyethyleneimine (PEI)-coated surface as model surfaces to study interactions between amino groups and serum complement pathway. SAMs of 11-mercaptoundecanol (OH-SAM) and 1-dodecanethiol (CH 3 -SAM) were used as control surfaces, respectively. Although much protein was adsorbed from serum solutions on the two types of amino surfaces, amounts of C3b deposition were much less than those observed on OH-SAM. Amounts of C3a released on the amino surfaces were same levels as that of CH 3 -SAM, but significantly smaller than that on OH-SAM. These facts suggest that the nucleophilic amino groups on NH 2 -SAM and PEI-coated surfaces do not directly activate the alternative pathway, but the protein adsorbed layers formed on amino surfaces activate it, but to an extent much smaller than that on OH-SAM. In addition, we found no deposition of C1q molecules on the amino surfaces, suggesting that these surfaces fail to activate the classical pathway. However, more careful studies are needed to conclude it, because it is known that C1q is only transiently detected at typical classical activation interfaces.