A number of independently isolated mutants of herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), selected for their inability to code for the enzyme deoxypyrimidine kinase (dPyK- mutants), have been analyzed for their ability to take part in interallelic complementation. Such interactions have been found not only between different mutants of the same strains of HSV-1, but also between mutants of different strains. dPyK- mutants of HSV-1 also complement a dPyK- mutant of HSV-2. The mutants have been classified into four groups by their genetic behaviour. In addition, mutants which take part in interallic complementation, on mixed infection with wild-type virus, interfere with the production of wild-type deoxypyrimidine kinase, while mutants which show no interactions have no such interfering properties. © 1978.