The interactions between biotin and avidin or streptavidin, a prototypical example of a specific biological ligand-receptor system, were studied by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Although this and other ligand-receptor systems have been studied by several techniques, including AFM, in this paper, a statistical analysis method which makes use of the properties of the Poisson distribution was applied, and the rupture strength of an individual interaction was obtained from the total adhesion forces measured by the AFM. Tip- and surface-modification chemistries were investigated by XPS and TOF-SIMS. The magnitudes of the interactions between biotin-avidin and biotin-streptavidin pairs, as determined by the Poisson method, were found to be 173 +/- 19 and 326 +/- 33 pN, respectively, for loading rates between 2 x 10(5) and 8 x 10(5) pN s(-1). These values are comparable to the values reported by other groups for the same systems. The statistical method used in this work has several advantages. It requires no assumptions about the surface energies or contact area between the AFM tip and the substrate, it is not limited by the force resolution of the instrument, and the number of measurements required to extract the individual unbinding force is significantly lower than that required by other methods.