A herpes-type virus that was originally isolated from a cell culture (designated K9V) derived from a tumor biopsy specimen from a patient with Kaposi's sarcoma was partially characterized. The host range of K9V, as determined by the induction of virus-specific cytopathology, synthesis of antigens, and plaque formation, was limited to human cells and particularly to fibroblasts. Immunofluorescence and complement fixation assays confirmed the specificity of the presence of cytomegalovirus (CMV)-type antigens in K9V-infected human fibroblasts. In addition, the density of K9V DNA was consistent with the density of CMV DNA. However, some peculiarities were observed in the K9V strain of CMV. The virus seemed more cell-associated in human fibroblasts than were known laboratory strains: The spread of cytopathology was slow and did not always involve the whole cell sheet, and the total regression of cytopathology with the establishment of a persistent infection was common. Similar characteristics have recently been observed in the Mj strain of CMV, which has been shown to be oncogenic in human fibroblasts.