SUMMARY When human rhinovirus type 2 attaches to HeLa cells, it first forms a reversible complex which can be dissociated by addition of EDTA to the medium. After prolonged incubation at 25 °C, most of the cell-associated virus progresses to an irreversible complex from which it cannot be eluted by EDTA, although some noninfectious material etutes spontaneously. The addition of a sufficient con- centration of SDS to block cell-mediated eclipse of the virus does not block the formation of the irreversible complex at 25 °C, while virus adsorbed and incu- bated at o °C remains reversibly bound. The number of receptor sites per cell is not influenced by SDS, but at o °C ceils appear to have fewer receptor sites than at physiological temperature.