DNA from a cosmid-cloned gene library of human cytomegalovirus (HCMV) strain AD169 was found to share sequence homologies with cellular DNA of various origins. Viral DNA fragments subcloned in plasmid vectors enabled localization of the homologous sequences to five regions of the HCMV genome. Four of these regions were found in the long unique (UL) segment of the viral genome (EcoRI fragments R, I and b, and HindIII fragment S); one region with virus--cell homology was located in the terminal inverted repeats (EcoRI fragments O and H). All hybridization reactions were carried out under stringent reannealing conditions (18 degrees C to 21.5 degrees C below average Tm of HCMV DNA). Fragments from these five HCMV DNA regions did not cross-hybridize with each other. The sequence homologies were seen in DNA from normal human placental tissue, liver autopsy material, intestinal tissue, white blood cells, and colon carcinoma cells, and in a series of haematopoietic cell lines. Sequence homologies were also detected in DNA from owl monkeys and Chinese hamsters, but not in mouse DNA. They were not related to human Alu repeats or cloned human c-myc sequences.