Eight r mutants, which are defective in fusion function, were isolated following BURR mutagenesis of a syncytial strain of herpes simplex virus type 2 (HSV 2). Most of the mutant pairs complemented each other and resulted in cell fusion, thus the r mutants could be classified into seven complementation groups. R mutants belonging to different complementation groups recombined effectively with each other, making it possible to construct a linkage map based on fusion function.