Adenovirus type 12 (Ad 12) infects baby hamster kidney cells (BHK21), but does not multiply in these cells. During a period of 114 hours after infection, replication of Ad 12 DNA cannot be detected by equilibrium sedimentation in CsCl density gradients, by zone sedimentation in sucrose density gradients or by DNA-DNA hybridization on membrane filters. The latter technique is the most sensitive one and allows detection of Ad 12 DNA when it constitutes ≥ 0.1% of the total intracellular DNA. Cellular DNA synthesis continues for at least 114 hours after infection. Starting between 24 and 50 hours after infection with Ad 12 at high multiplicity, cellular DNA fragments of 5-10 × 106 daltons in molecular weight are observed. They are conceivably due to an endonuclease cleaving double-stranded DNA. Adenovirus type 2 (Ad 2) multiplies in BHK21 cells, and replication of Ad 2 DNA begins between 14 and 16 hours after infection. Cellular DNA synthesis is turned off approximately 17 hours after infection. When BHK21 cells are doubly infected with Ad 2 and Ad 12 simultaneously, only Ad 2 DNA synthesis can be detected. © 1969.