Plasminogen activator is produced by hamster cells transformed by human herpesviruses. These cell lines have previously been shown to be oncogenic when injected s.c. into newborn syngeneic hamsters. Lysis of fibrin overlays by these cell lines was plasminogen dependent. Normal hamster embryo fibroblasts and a hamster cell line transformed by PARA-7 (an adenovirus-SV 40 hybrid) failed to produced lysis. In separate experiments fibrin overlay of lytically infected secondary rabbit kidney cells did not show induction of this activity during the normal course of productive infection. The human cell line TE-85 clone F-5, a clonal cell line from a human osteogenic sarcoma, failed to produce plasminogen activator, but two separate clones of these cells that were morphologically transformed after exposure to UV-inactivated herpes simplex virus type 2 produced rapid lysis of the fibrin overlay. Clonal variation was observed in herpes simplex virus types 1 and 2-transformed hamster lines and is under investigation. It is suggested that plasminogen activator detection may serve as a convenient assay system for transformation of normal cells by herpesviruses.