Summary Cells infected with temperature-sensitive mutants of adenovirus type 5 have been labelled with [35S]-methionine and extracts examined by polyacrylamide gel electrophoresis followed by autoradiography. With the exception of one group of mutants, cells pulse-labelled for short periods (< 1 h) showed similar patterns of polypeptide synthesis whether they were infected with wild-type virus or with the mutants. However, on longer labelling it was evident that certain polypeptides were relatively less well labelled in the cells infected with the mutants at the restrictive temperature. A pulse-chase experiment suggested that preferential degradation of these polypeptides was occurring presumably by a proteolytic surveillance mechanism which could recognize defective polypeptides. One group of mutants showed no synthesis of capsid polypeptides at the restrictive temperature, in agreement with the results of previous serological investigations.