The characteristics of a human cytomegalovirus (HCMV) transformed syrian hamster cell line (87-TRH-5) were examined. The cytomegalovirus DNA (HCMV-DNA) labelled in vitro by nick translation was used as a probe to detect viral DNA sequences in the 87-TRH-5 line. Cytomegalovirus specified DNA sequences were detected in cells examined up to passage 58, but were undetectable at higher passage levels. Six clones were derived from the 28th passage of 87-TRH-5 cells and examined for HCMV-DNA sequences. Various amounts of HCMV-DNA were found. A tumour induced by passage 28 cells contained no detectable HCMV-DNA sequences.