Solid phase-supported receptor and ligand competitive assays for biotin were tested with respect to a mathematical model. We could assume the same mathematical model for both the solid phase-supported receptor and ligand competitive assays. In addition, we could eliminate the so-called blocking procedure in these assays. The amount of immobilizing reagent (biotinylated bovine serum albumin, BBSA) in the solid phase-supported ligand competitive assay was less than that (avidin) in the solid phase-supported receptor competitive assay. Desthiobiotin and biocytin influenced the solid phase-supported ligand competitive assay, but lipoic acid and lysine did not.