Thirteen ts mutants of type 2 herpes simplex virus were backcrossed to a syncytial but not temperature-sensitive mutant of wild-type virus. This was an attempt to introduce a third marker, syncytial plaque morphology or syn, into at least some of the ts mutants. Three ts mutants carrying the syn marker were obtained but only one, ts 9, was satisfactory for genetic experiments. Three-factor crosses were carried out between ts 9 syn and the mutants which determined the order of eleven ts mutations relative to both the ts 9 mutation and the syn mutation. A provisional linkage map based both on the order derived from the three-factor crosses and on map distances from recombination frequencies has been prepared: it contains nine ts mutations and the syn mutation.