SummaryMicrobiological experiments involving direct assays as well as bioautographic procedures indicate that human urine contains relatively large amounts of free bio-tin and very small amounts of a “hydrophilic” biotin derivative that may be “biocytin sulfoxide.” No evidence was found for the existence of other “unusual” biotin derivatives. All the biotion of urine determinable by an organism more versatile in its response to biotin derivatives than any studied previously is avidin combinable.