Summary Inoculation of newborn hamsters with virus doses greater than 103 TCD50 of types 1 or 2 Herpes virus hominis (HVH) strains caused a mortality approaching 100%. With lesser virus doses, nine sarcomas have been found 5–28 months after inoculation of four different noninactivated HVH type 2 strains and one other ultraviolet-inactivated type 2 strain. None of the animals inoculated with HVH type 1 strains developed tumors, and one newborn hamster inoculated intrathoracically with Eagle's MEM developed a cheek-pouch well-differentiated fibrosarcoma. Eight of the nine tumors with HVH type 2 strains were at or close to the site of inoculation and the histological characteristics of some of these tumors differ from those associated with other viral-induced hamster sarcomas. No clear-cut evidence of a relationship between HVH and the tumors by a variety of virological and serological assays has been obtained as yet. The finding of C-type particles in two of three tumors or their explants by electron microscopy has raised further questions about the role of HVH in the causation of the tumors found.