Human milk has been shown to contain heterogeneous oligoribonucleotides varying in size from dimers to 100 mers. The sets of long oligonucleotides in milk samples from different donors and from different stages of lactation have some conservative elements. Sequences of some RNA oligonucleotides correspond to the 3'-part of 5.8S human ribosomal RNA and to 3'-parts of tRNAVal and tRNATyr. Primary structures of other oligo-RNAs are homologous to fragments of human 18S and 28S rRNAs. Concentration of RNA in milk samples tends to decrease with lactation development. The change of RNA concentration and the low concentration of DNA in milk plasma suggest that the source of RNA in human milk may be active secretion of RNA by mammary gland cells.