Thirty-nine temperature-sensitive (ts) mutants that fail to grow at 39.5 degrees C but develop normally at 33 degrees C have been isolated from a nitrous-acid-treated stock of a wild-type strain of type 2 human adenovirus. The frequency of ts mutants among the surviving viruses was about 10%. Complementation tests in doubly infected cell cultures at restrictive temperature permitted the assignment of 19 of these mutants to 11 complementation groups. They are characterized phenotypically according to their soluble capsid antigen production quantified by two-dimensional immunoelectrophoresis, virus DNA synthesis, as measured by alkaline sucrose gradient sedimentation of 34S DNA, and virion morphogenesis, as analysed by electron microscopy os cell sections. Two complementation groups were defective for DNA synthesis, four for soluble hexon production and two groups for total penton (penton base + fibre), while one group revealed no fibre production. Two complementation groups presented a normal antigen pattern, but the particles exhibited altered morphology as observed in cell sections.