A simple, universal, and rapid enzymatic method for the quantitative determination of cell adhesion in 96-well cell culture plates has been established. The assay is based on cellular steady-state endocytosis, which is used to label cells with horseradish peroxidase (HRP) prior to adhesion. Subsequently, attached cells can be detected by a simple enzymatic reaction, in which the accumulated HRP catalyzes dye formation from a colorless hydrogen donor, e.g., o-phenylenediamine, in the presence of hydrogen peroxide. As demonstrated with different cell lines and test systems, the method can be used to quantify cell-matrix as well as cell-cell interactions and allows a very sensitive quantification of adherent cells. The HRP label is nontoxic and does not affect the adhesion properties of tested cell lines; the quantity of dye formed is proportional to the number of adherent cells. Furthermore, the assay represents an alternative method to isotopic cell labeling, e.g., with 51Cr, which is usually used for quantifying cell-cell interactions.