Here we present a procedure for quantifying single protein molecules affixed to a surface by counting bound antibodies. We systematically investigate many of the parameters that have prevented the robust single-molecule detection of surface-immobilized proteins. We find that a chemically adsorbed bovine serum albumin surface facilitates the efficient detection of single target molecules with fluorescent antibodies, and we show that these antibodies bind for lengths of time sufficient for imaging billions of individual protein molecules. This surface displays a low level of nonspecific protein adsorption so that bound antibodies can be directly counted without employing two-color coincidence detection. We accurately quantify protein abundance by counting bound antibody molecules and perform this robustly in real-world serum samples. The number of antibody molecules we quantify relates linearly to the number of immobilized protein molecules (R2 = 0.98), and our precision (1-5% CV) facilitates the reliable detection of small changes in abundance (7%). Thus, our procedure allows for single, surface-immobilized protein molecules to be detected with high sensitivity and accurately quantified by counting bound antibody molecules. Promisingly, we can probe flow cells multiple times with antibodies, suggesting that in the future it should be possible to perform multiplexed single-molecule immunoassays.