The fatty acid contents of thirteen commercial preparations of human albumin were found to be in the range 0.03 to 9 mol of fatty acid/mol albumin. Marked differences were found between the preparations in the binding of the fluorescent probes, 5-dimethylamino-naphthalene-1-sulfonamide (DNSA) and dansylsarcosine. The displacement of these probes by ibuprofen and phenylbutazone also showed marked differences between preparations. The differences between the commercial albumin samples correlated well with their fatty acid contents and were abolished by treatment with charcoal. They were similar to the changes observed when oleic acid was added to fatty acid free albumin. The source and fatty acid content of commercial albumin preparations should be considered when comparing studies of the binding of drugs to human albumin.