A few that contain no virus detectable by immunofluorescence or electron microscopy [e.g., Raji cells derived from a Burkitt lymphoma (Pulvertaft, 1965) , NHDL 3 cell cultures from normal human leukocytes (Gerber and Monroe, 1968) , NC 37 from leukocytes of a patient with pneumonia (see Gerber, 1972) ] have been shown to contain cryptic virus by cloning (Maurer et ai, 1969) , nucleic acid hybridization (Zur Hausen andScholte-Holthausen, 1970), complement fixation (Vonka et al., 1970) , or activation by bromodeoxyuridine (BrdU) (Gerber, 1972) .