In this study we describe DNA-RNA complexes in matrix DNA of Friend cells. The presence of such unusual structures is confirmed by the following evidence. When a preparation of matrix DNA is electrophoresed in agarose an RNA component always migrates together with DNA. There should be a close interaction between DNA and RNA in such a preparation because the presence of the RNA component causes resistance of DNA to DNase I and Exo III. An intimate, hybrid-type association of part of the RNA component with DNA is indicated also by the fact that about 20% of this RNA is sensitive to RNase H. By specific inhibition of the RNA synthesis with alpha-amanitin and actinomycin D it was shown that the bulk of associated RNA is transcribed by RNA polymerase III. Hybridization experiments showed similarity between the DNA sequences isolated from the complexes and those from the base of dehistonized DNA loops obtained by high-salt extraction of nuclei. This observation suggests that the complexes might represent attachment sites of nuclear DNA to the matrix: possibly, the attachment is mediated via the RNA component. Experiments with induction of erythroid differentiation indicated that a profound reorganization of the nucleus, accompanying terminal differentiation, leads to a striking reduction in the number of complexes and thus in the number of attachment sites. This suggests that the complexes should function as transient attachment sites.