Abstract A system has been described of producing HeLa and MAS cell monolayers on which adenoviruses types 4 and 5 produce well-defined plaques. The first visible plaques appear on the eighth day, and the number increases at a regular rate until after about 3 weeks they may reach ten times the original number. There is a linear relationship between virus input and number of plaques obtained. When adenovirus is put on these plates about half the virus attaches within the first 60 minutes. The first new virus appears at 18 hours, reaching a maximum in 36 hours with a yield of approximately 10 4 PFU per cell. All but a few per cent of the virus remains cell attached for as long as 72 hours. At high inputs (multiplicity 80), 90% of the cells become infective centers and the number of colony formers drops to 0.1%.