A temperature-sensitive mutant of adenovirus type 2 grown at 39 degrees (tsl-39 degrees) produces noninfectious physical particles. DNA and core structures derived from such virion particles, however, were found to be infectious at the permissive temperature. Calcium chloride mediated transfection showed that core structures were 20-40 times more infectious than deprotenized DNA. Because in vivo the noninfectious ts1-39 degrees virions are blocked in uncoating at the core-like stage, we attempted to determine the reason for the infectivity of tsl-39 degrees cores prepared in vitro by investigating the metabolic fate of labeled cores in transfected cells. As opposed to infection with virus, the tsl-39 degrees cores were found to penetrate the nucleus efficiently and uncoat by shedding their core proteins. A possible explanation for the infectivity of the cores is discussed.