Biological, histological, and ultrastructural studies were made on tumors produced in weanling hamsters inoculated with hamster embryo fibroblasts (HEF) transformed by herpes simplex virus type 2 or with cultured cells (tumor cell lines T1 and T2) derived from the tumor tissues. The malignancy of transformants increased through passages in vitro and in vivo. Histologically, two tumor cells lines (155-4T1 and U-15T1) produced fibrosarcomas and one (U-26T1) produced lesions resembling "malignant fibrous histiocytomas." Ultrastructurally, fibrosarcomas produced by 155-4T1 and U-15T1 consisted mainly of fibroblast-like cells with extracellular collagen fibers, whereas malignant fibrous histiocytoma-like lesions produced by U-26T1 consisted of undifferentiated cells, multinuclear giant cells, histiocyte-like cells, and fibroblast-like cells. Frequently, various kinds of "nuclear bodies" were found in the nuclei of tumor cells. Several herpes virus-like particles (120 approximately 140 nm in diameter) were detected in some nuclei of undifferentiated tumor cells produced by U-26T1.