Enzymes can be immobilized at a solid–liquid interface and observed with a scanning tunneling microscope (STM) using the inherent charge of proteins and an external potential applied to the STM substrate. Phosphorylase kinase, and dimers and oligomers of phosphorylasebhave been observed at the interface of aqueous solutions and highly oriented pyrolytic graphite (HOPG). The lateral dimensions of phosphorylase kinase determined by STM at the solid–liquid interface are from 74%–78% of the dimensions determined by STM at the solid–air interface [Biochem.28, 4939 (1989); Biophys. J. (in press)]. The phosphorylaseblateral dimensions of both enzymes are between 1.9 and 2.5 nm greater than the dimensions determined by x‐ray crystallography. The vertical dimensions determined by STM at the solid–liquid and solid–air interfaces are in reasonable agreement with each other. Mixtures of the two enzymes show aggregates in which the complexes of the two enzymes are identifiable. This technique will make it possible to use STM to study the structure of protein systems at nearly physiological conditions and the nature of the interaction of proteins with surfaces.