BALB/c-derived 10E2 cells were made thymidine kinase(TK)-negative and one isolated clone (B2) was used for studying morphological and biochemical transformations by herpes simplex virus (HSV) type 1 (strain 10412). The B2 cells displayed a "normal" flat appearance and were nontumorigenic in nude mice when tested at frequent intervals over a period of 45 subcultures. B2 cells infected with UV-irradiated HSV (UV-HSV) and maintained in normal growth medium showed foci of spindle-shaped cells after one subculture. The cells from these morphologically transformed foci were tumorigenic in nude mice and were TK negative. B2 cells infected with UV-HSV or transfected with the HSV-1 TK gene and maintained in TK-selective medium showed discrete colonies of cells which displayed a normal flat appearance and expressed the viral TK enzyme. These biochemically transformed B2 cells were nontumorigenic in nude mice. The findings with B2 cells indicate that biochemical and morphological transformations by HSV-1 are independent events and suggest that the HSV-1 TK gene is a suitable vehicle for introducing non-TK genes into cells to assess their transforming potential.