Monoclonal antibodies to biotin have been prepared by using biotin linked to keyhole limpet haemocyanin (KLH) as the antigen. Spleen cells obtained from mice immunized with biotin-KLH were fused with the myeloma cell line NS-1. The resulting hybridomas were screened for the production of antibodies to biotin using an enzyme-linked immunosorbent assay. Clones producing antibodies to biotin were isolated by limiting dilution methods. Four cell lines, each derived originally from a different fusion, were chosen for the production of monoclonal antibodies. The monoclonal antibodies obtained have been characterized with respect to their ability to interact with biotin, biotin-bovine serum albumin, biotin-KLH and biocytin as well as to inhibit biotin-dependent enzymes. They have been used to produce cellular biotin deficiency in vitro for studies of biotin function.