Cosmids containing large fragments of herpes simplex virus type 1 DNA were prepared using a vector that allows intact inserts to be excised using the restriction endonuclease PacI. Two independent sets (A and B) of five cosmids were identified whose inserts overlap and represent the entire viral genome, and set C was obtained by replacing two cosmids in set B. Each set gave rise to viral plaques when digested with PacI and transfected into cells in culture. Two cosmids common to sets B and C ostensibly contain one of the origins of viral DNA replication (oriL) in a region of overlap between inserts, but both actually consist of a minority of apparently intact (ori+L) forms and a majority of deleted (ori-L) forms. These sets yielded exclusively ori+L viral progeny. When either of these cosmids was replaced by a derivative comprising only ori-L forms, ori+L and ori-L progeny were obtained, and only ori-L progeny were produced when both were replaced. One cosmid in set A contains the oriL locus in a nonoverlapping region and lacks ori+L forms. This set generated only ori-L virus. Viral mutants with lesions in either or both of genes UL2 and UL44, which are not essential for growth in cell culture, were constructed using cosmids containing specifically introduced frameshift mutations. A mutant with a frameshift mutation in an essential gene (UL33) was isolated by transfecting a complementing cell line. These results indicate that a cosmid-based system will facilitate isolation of large numbers of defined viral mutants.