Culture fluid of human epitheloid (HEp-2) cells was examined for extracellular lactate dehydrogenase activity as an indicator of cell damage during a 48 h period in which virus replication and changes in cell morphology occurred. Uninfected and adenovirus type 5-infected cells had the same levels of extracellular enzyme activity both before and after the appearance of morphological changes in cells due to virus infection, whereas adenovirus type 12-infected cells showed increased extracellular enzyme activity. Cells infected with either adenovirus type 5 or type 12 had the same total cellular and extracellular lactate dehydrogenase activity. Hydrocortisone, a membrane stabilizing agent, prevented abnormal leakage of lactate dehydrogenase from adenovirus type 12-infected cells, but had no effect on virus replication or total enzyme activity of infected cells. After inoculation of monkey kidney (Vero) cells the yield of progeny adenovirus type 5 virions was greatly reduced and there was no production of adenovirus type 12 virions. The pattern of extracellular lactate dehydrogenase activity of uninfected and adenovirus type 5- and type 12-infected Vero cells was like that with HEp-2 cells. Therefore, production of adenovirus type 12 virions is not necessary for the virus-cell interaction causing cell membrane labilization.