The matrix attachment regions of the chicken lysozyme domain were studied in an in vitro DNA binding assay by incubating oviduct nuclear matrices with labeled restriction fragments. A strong attachment region was localized between 11.1 and 8.85 kb upstream of the transcription start site and a weaker one between 1.3 and 5.0 kb downstream of the poly(A)+ addition site. Both attachment regions co-map with the previously established boundaries of the chromatin domain. The upstream matrix attachment region is distinguishable from known enhancers and is composed of multiple binding sites. We find specific but weaker binding of the same restriction fragments to matrix preparations from transcriptionally inactive chicken erythrocytes indicating a cell-type and transcription-independent conservation of the sites for specific binding of matrix attachment sequences. We also demonstrate that the matrix attachment regions are located at the base of a chromosomal loop in histone-extracted nuclei. Thus, the lysozyme domain represents a topologically-sequestered functional unit containing the coding region and all known lysozyme-specific, cis-acting regulatory elements.